CBSE Notes

By going through these CBSE Class 11 Biology Notes Chapter 1 The Living World, students can recall all the concepts quickly.

The Living World Notes Class 11 Biology Chapter 1

→ There is lots of variety in the living world i.e., flora and fauna. They exhibit distinctive characteristics like growth, reproduction, metabolism, etc.

→ In the past, man could not perceive the difference between inanimate matter and living organisms.

→ A common feature of inanimate and animate objects was the sense of awe that they evolved.

→ Biologists indulged in the anthropocentric view of biology could register limited progress in biological knowledge.

→ The curiosity to study life forms in detail bought in systems of identification, nomenclature, and classification.

→ The biggest gain from these studies was the recognition of the sharing of similarities among living organisms.

→ The conservation of biodiversity started when a man came to know that all living organisms are related to one another.

→ The wide range of living types is amazing. Living organisms are found in all types of habitats be its cold mountains, deciduous forests, oceans, freshwater lakes, deserts or hot springs, etc.

→ Growth, reproduction, metabolism, self-replicate self-organize, interaction, and the ability to sense the environment are some unique features of living organisms.

→ In order to study the diversity of organisms a new branch of biology is evolved which is called Taxonomy, Taxonomists have developed inter-national codes, Taxonomic aids, and keys that help them in identification naming, and classification of organisms.

→ Taxonomic studies in various species of plants, animals, etc. are useful in important fields such as agriculture, forestry, industry, and in knowing our bioresources and their diversity.

→ With the development of science and technology, biologists have established certain procedures and techniques to store and preserve the information of different species which will be helpful to understand and study them.

→ Classification: The method of systematically arranging the dif¬ferent species of living organisms on the basis of similarities and differences in various groups is called classification.

→ Taxonomy: The branch of science which deals in the study of the diversity and kind of organisms and the evolutionary relationships among them.

→ Species: refer to a group of organisms that resemble each other interbreed and produce fertile off-springs.

→ Genus: A group of related species which has more characters in common in comparison to species of other genera.

→ Family: A group of the related genres with still fewer similarities as compared to genus and species.

→ Order: Categories like species, genus, and families are based on a number of similar characters. This is referred to as order.

→ Class: Category which includes related orders.

→ Phylum: A higher category that consists of classes comprising of animals such as fishes, amphibians, reptiles, birds along with mammals.

→ Kingdom: The highest category which consists of all animals belonging to various phyla.

→ Botanical garden: Specialised gardens having collections of living plants for reference.

→ Taxonomic keys: Tools that help in identification based on characteristics.

By going through these CBSE Class 11 Chemistry Notes Chapter 14 Environmental Chemistry, students can recall all the concepts quickly.

Environmental Chemistry Notes Class 11 Chemistry Chapter 14

→ Environmental Pollution: Atmospheric, Tropospheric & stratospheric.

→ Water Pollution: Causes of water pollution & International standards of drinking water.

→ Soil Pollution: Pesticides-Intecticides & Herbecides & Fungicides

→ Industrial wastes: Recycling of wastes

→ Strategies to Control Environmental Pollution: Waste management.

→ Green Chemistry: Green Chemistry in day to day life.

→ Environmental Pollution: Environmental pollution is the effect of undesirable changes in the surrounding that have harmful effects on animals & plants & human beings.

→ Atmospheric pollution: Tropospheric & stratospheric.

→ Global warming: About 75 % of solar energy reaching the earth is absorbed by the earth’s surface, which increases its temperature. The rest of the heat radiates back to the atmosphere. Some of the heat is trapped by gases such as CO₂, CH₄ & O₃ & C.F.Cs. & water vapours in the atmosphere. Thus they add to the heating of the atmosphere. This causes Global Warming.

→ Green House Effect: Atmosphere traps the sun’s heat near the earth’s surface & keeps it warm. This is called the Greenhouse effect because this makes the earth perfect for life.

→ Acid Rain: When the pH of rainwater drops below 5.6, it becomes acidic. Acid rain is caused by the presence of oxides of N & O in the atmosphere.

Acid rain is harmful to agriculture, trees & plants & washes away nutrients needed for their growth,

→ Classical Smog: It is a mixture of smoke, fog & SO₂ chemically it is a reducing mixture & so it is called reducing smog.

→ Photochemical smog: The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons & nitrogen oxides produced by automobiles & factories. Photochemical smog has a high concentration of oxidizing agents & is, therefore, called oxidizing smog.

→ Ozone hole: In October 1979, A zone of lowered ozone concentration was detected over Antarctica which occurs mainly during September-October & gets replenished during November-December. This is called the ozone hole.

→ B.O.D.: Biochemical oxygen demand.

→ C.O.D.: Chemical oxygen demand.

→ Green Chemistry: It involves processes & products that reduce or eliminate the use or the generation of hazardous substances.

Chapter In Brief:
Environment Pollution:
It is defined as the effect of undesirable changes in our surroundings due to natural sources or human activity that have harmful effects on plants, animals and human beings. A.substance, which causes pollution to one or more components of the ecosystem are called pollutants. Pollutants can be solid, liquid, or gas.

Pollutants are of two types.
1. Non-Biodegradable Pollutants: The materials which do not undergo degradation or degrade very slowly in the environment like DDT, plastic materials, heavy metals, many chemicals, nuclear wastes etc.

2. Biodegradable Pollutants: The materials which are easily decomposed by the micro-organisms either by nature or by suitable treatment are called biodegradable pollutants. Examples; Domestic waste like discarded vegetables, cow dung etc.

→ Atmospheric Pollution: The lowest region of the atmosphere in which human beings along with other organisms live is called the troposphere. It extends up to the height of- 10 km from sea level. Above the troposphere between 10-15 km above sea level lies the stratosphere. Atmospheric pollution is generally studied as tropospheric and stratospheric pollution.

→ Tropospheric Pollution: It occurs due to the presence of undesirable solid or gaseous particles in the air.
1. Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone etc.

2. Particulate pollutants: These are dust, mist, fumes, smoke and smog”.
1. Gaseous air pollutants
(a) Oxides of sulphur are produced when sulphur-containing fossil fuel is burnt. SO₂ is poisonous to both animals and plants. Particulate matter present in polluted air catalyses the oxidation of SO₂ to SO₃.
2SO₂(g) + O₂(g) → 2 SO₃(g)

The reaction can also be promoted by ozone and hydrogen peroxide.
SO₂(g) + O₃(g) → SO₃(g) + O₂(g)
SO₂(g) + H₂O₂ → H₂SO₄

(b) Oxides of Nitrogen: At high altitudes when lightning strike’s N₂ and O₂ present in the air combine to form nitric oxide.

Rate of formation of NO₂ Then NO reacts with O₃ is faster.
NO(g) + O₃(g) → NO(g) + O₂(g)
The irritant red haze in the traffic and congested places is due to oxides of nitrogen. NO₂ is a lung irritant.

(c) Hydrocarbons are formed by incomplete combustion of fuel used in automobiles. They are carcinogenic.

(d) Oxides of carbon
1. Carbon monoxide (CO) is one of the most serious air
pollutants. It is highly toxic. It blocks the delivery of oxygen to the organs and tissues. It is produced as a result of incomplete combustion of carbon
C(s) + \(\frac{1}{2}\)O₂(g) → CO(g)
Insufficient quantity

It binds to haemoglobin for which it has 200-300 times more affinity than oxygen and forms carboxy-haemoglobin thus the oxygen-carrying capacity of blood is greatly reduced.

2. Carbon dioxide (CO₂): It is released into the atmosphere by respiration and burning of fossil fuels. It is also emitted during volcanic eruptions and limestone burning to produce lime for cement plants. Normally a percentage of CO₂ in the atmosphere is only 0.03 by volume. The increased amount of CO₂ in the air is mainly responsible for global warming.

Global Warming and Greenhouse Effect:
Some of the heat from solar energy is trapped by gases such as carbon dioxide, methane (CH₄), ozone, chlorofluorocarbons compounds (CFCs) and water vapours in the atmosphere. They add to the heating of the atmosphere. This causes global warming.

The natural greenhouse where there is an ecological balance of CO₂, water vapours which are continuously being converted to carbohydrates and a fresh supply of oxygen during photosynthesis by a lot of green plants, flowers, vegetables makes the earth perfect for life. Sun’s warmth is retained on the earth giving sufficient heat to the soil and plants which emit infrared radiations.

If the amount of CO₂ crosses the delicate 0.03% the natural greenhouse balance is disturbed. It is a major contributor to global warming. CFCs are damaging the ozone layer. Other contributors to global warming are CH₄, N₂O, and ozone. Cutting down forests and trees, burning fossil fuels, production of fertilizers, man-made industrial chemicals etc. add to global warming.

Acid Rain:
Due to the presence of CO₂ in the atmosphere, the pH of rainwater is 5.6.
H₂O(l) + CO2(g) ⇌ H₂CO₃(aq)
H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃ (aq)

When the pH of rainwater falls below 5.6, it is called Acid Rain Various human activities release oxides of Nitrogen and Sulphur in the atmosphere. SO₂ and NO₂ are thus major contributors to acid rain.
2SO₂(g) + O₂(g) + 2H₂O(l) → 2H₂SO₄ (aq)
4NO₂(g) + O₂(g) + 2H₂O(g) → 4HNO₃(aq)

Acid rain is harmful:

1. For plants, trees and agriculture as it washes away nutrients needed for their growth.
2. For human beings and animals as it causes respiratory ailments.
3. An aquatic ecosystem is disturbed when this add rain reaches water objects like rivers, lakes etc.
4. It corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into drinking water.
5. Acid rain damages buildings and other structures made of stone or metal. The Taj Mahal is being corroded by acid rain.
   CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

As a result, the beautiful white marble of which the Taj Mahal was built centuries ago is rendered lusterless and is being slowly eaten away.

Particulate Pollutants:
They are of two types.
1. The Viable Particulates: They are bacteria, fungi, moulds, algae etc. which are minute living organisms that are dispersed in the atmosphere. They cause allergy to human beings and bring disease to plants.

2. Non-Viable Particulates: They are classified according to their nature and size as follows:
(a) Smoke particulates are solid or a mixture of solid and liquid particles formed during combustion of organic matter,
(b) Dust particles. Over 1 mm in diameter produced during the crushing, grinding and attribution of solid materials, sawdust, sand and cement particles, fly ash, dust storm etc. are the typical examples of this type of particulate emission.
(c) Mists are produced by particles of spray liquids and by condensation of vapours in air, Examples are H2S04 mist, herbicides and insecticides.
(d) Fumes coming out from chemical plants, sublimation, distillation, generally organic solvents, metals and metal oxides form fume particles.

The effect of particulate pollutants are largely dependent on particle size, They are all dangerous to human health.

Smogs:
The word smog is derived from smoke and fog. This is the major cause of air pollution.

Smogs are of two types:
(a) Classical Smog
(b) Photochemical Smog

(a) Classical Smog: It occurs in a cool humid climate. It is a mixture of smoke, fog, and SO2. Because of its reducing nature, it is also called Reducing Smog.

(b) Photochemical Smog: It occurs in a warm, dry and sunny climate. The main components result from the action of sunlight on unsaturated hydrocarbons and oxides of nitrogen. Since it has a high concentration of oxidizing agents, it is called Oxidizing Smog.

Formation of Photo Chemical Smog:
When both unsaturated hydrocarbons (unburnt fuels) and nitric oxide (NO) build-up to sufficiently high levels, a chain reaction occurs in the presence of sunlight.

Ozone is a toxic gas. Both NO₂ and O₃ are oxidising agents.

They further produce chemicals like acrolein (CH₂ = CH CHO) and peroxyacetyl nitrate (CH₃ CO₃ NO₂) or PAN

→ Effects of Photochemical Smog: Photochemical smog causes serious health problems. Both O₃ and PAN present in photochemical smog cause extreme irritation to the eyes. O₃ and NO irritate nose and throat and their high concentration causes a headache, chest pain, dryness of throat, cough and difficulty in breathing. It causes extensive damage to plant life, causes corrosion of metals, building materials, cracking of rubber etc.

→ Control: Photochemical smog can be controlled by controlling—the production of oxides, of nitrogen, controlling the burning of fossil fuels, using catalytic converters in automobiles etc. Certain plants also metabolise NO and therefore their plantation can help.

Stratospheric Pollution
Formation and Breakdown of Ozone: A large amount of ozone (O₂) is present in the stratosphere. O₃ protects us from the harmful UV rays coming from the sun.

→ Reactions occurring in the stratosphere: The main reactions occurring in the stratosphere. In this region ozone is formed in two steps: In the first step ultraviolet radiation coming from the sun have sufficient: energy to split dioxygen into two oxygen atoms. In the second step, the oxygen atoms react with more dioxygen to form ozone.


This ozone absorbs ultraviolet radiation and breaks down into dioxygen and an oxygen atom. Heat is given off, which warms up the stratosphere. Thus there is a dynamic equilibrium between the production and decomposition of ozone molecules.

This ozone layer is acting like a protective layer for the life on the earth. The following reactions take place in this context:

Freons are introduced in the atmosphere from aerosol sprays in which they function as propellants and from refrigerant equipment in which they act as coolants:

This is reactive chlorine destroys the ozone through the following reactions:

The Ozone Hole:
The depletion of the ozone layer by two major compounds [NO₂ and CH₄] is called the ozone hole.

In the summer season, NO₂ and CH₄ react with chlorine monoxide (CIO₂) and chlorine atoms (Cl) forming chlorine sinks, preventing much ozone depletion.

Whereas in winter, a special type of clouds, called polar stratospheric clouds are formed over Antarctica. On the surface of these clouds, chlorine nitrate (CIONO₂) formed gets Hydrolysed to form hypochlorous acid. It reacts also with HCl (above) to give Cl₂
CIONO₂(g) + H₂O(g) → HOCl(g) + HNO₃(g)
CIONO₂(g) + HCl(g) → Cl₂(g) + HNO₃(g)

When sunlight returns to Antarctica in the spring, the sum’s warmth breaks up the clouds and HOCl and Cl₂ are photolysed to gaseous chlorine atoms.

The chlorine-free radicals thus formed, initiate the chain reaction for ozone depletion. One CFC molecule destroys one lac molecules of O₃.

Effects of Depletion of Ozone Layer:

1. The most serious effect is the development of an ozone hole which will allow UV radiations from the sun to pass through the stratosphere and reach the earth causing among other ailments skin cancer.
2. UV rays cause; damage to the cornea and lens of the eye and may cause cataract or even blindness. ,
3. It affects plants chlorophyll, proteins, and causes harmful mutation.
4. O₃ depletion will affect the climate badly. It will upset the heat balance of the earth.
5. It will lead to ecological imbalance which will adversely affect both man and animal.

Water Pollution: Major Water Pollutants

Causes of Water Pollution:
1. Pathogens: They are disease-causing agents. They include bacteria and other organisms that enter the water from domestic sewage and animal excreta. Human excreta contain bacteria that cause gastrointestinal diseases.

2. Organic Wastes: The other major water pollutant is organic matter such as leaves, grass, trash etc. Excessive phytoplankton growth within the water is also a cause of water pollution. These are biodegradable. Decomposition of organic matter by bacteria in water causes depletion of dissolved oxygen in the water which is very essential for aquatic life.

3. Chemical Pollutants: Water-soluble inorganic chemicals like cadmium, nickel and nickel salts are an important class of water pollutants. They are dangerous to human life because our body cannot excrete them. Slowly and slowly they damage kidneys, central nervous system, liver etc. Petroleum products like oil spills in oceans, polychlorinated biphenyls (PCBs), though biodegradable are another source of organic chemicals that pollute water.

International Standards For Drinking Water:

1. Fluoride: A deficiency of fluoride in water for drinking is harmful to man and causes tooth decay. Soluble fluoride is added to drinking water to bring its concentration to 1 ppm. Its concentration in water is more than 2 ppm and has harmful effects on teeth, bones.
2. Lead: The upper prescribed limit for the presence of lead in water is 50 ppb. Lead damages the kidney, liver and reproductive system.
3. Sulphate: Excessive sulphate (> 500 ppm) in drinking water causes a laxative effect. At moderate levels it is harmless.
4. Nitrate: The maximum limit of nitrate in drinking water is 50 ppm. Excessive presence can cause disease such as methemoglobinemia (blue baby syndrome)
5. Other metals: The maximum concentration of some common metals recommended in drinking water are given below:

Maximum Prescribed Concentration of Some Metals in Drinking Water:

Soil Pollution
Insecticides, pesticides and herbicides used for the protection of crop cause soil pollution.

Pesticides
Commonly used pesticides which are toxic substances are DDT, Aldrin, and Dieldrin. When their concentration increases beyond a limit, they cause serious metabolic and physiological disorders in higher animals. They are water-insoluble and non—biodegradable. With the passage of time, insects & pests have become immune to these pesticides and insecticides. More potent and biodegradable products like organophosphates and carbonates have beef put to use.

These chemical are extremely harmful to humans and agriculture felid work who spray them.

Nowadays, the pesticide industry is using Herbicides such as sodium chlorate (NaCIO₃), sodium arsenite (Na₃AsO₃) and many others. But all of them are not environmental friendly. Most herbicides are toxic to mammals,

Industrial Waste:
Industrial solid wastes are sorted out as biodegradable and non- Biodegradable wastes. The former is generated by cotton mills, food processing units, paper mills and textile factories, whereas the latter is generated by thermal power plants, iron and steel plants, industries producing Al, Zn and Cu.

The disposal of non-degradable industrial solid waste should be done suitably, otherwise, it may cause a serious threat to the environment. There should be proper management of both domestic and industrial wastes.

Green Chemistry:
Green chemistry as an alternative tool for reducing pollution: One way to protect our environment from chemical effluents and waste is to use green chemistry. By green chemistry, we mean producing the chemicals of our daily needs using such reactions and chemical process which neither use toxic chemicals nor emit such chemicals into the atmosphere. Although it is a very challenging task.

Green chemistry does not employ toxic reagents and severe conditions but uses mild and environmentally friendly reagents, such as sunlight, microwaves, sound waves and enzymes. The use of sunlight and ultraviolet radiation is very useful to produce some useful products which cannot be obtained by simple chemical reactions microwaves and sound waves are used for chemical reactions giving extraordinary results which are not possible by simple chemical reactions.

Enzymes are environmentally friendly reagents. These work in aqueous solutions and at ambient temperatures. These methods are used for preparing medicines and certain antibiotics. For examples, semi-synthetic penicillins such as ampicillin, and Amoxycillin, have been prepared by using this technique.

By using these methods, green chemistry will help us to keep our environment pollution-free.

Green Chemistry in Day-to-Day Life:
1. Dry cleaning of clothes: In places of tetrachloroethene (C₁₂C = CC₁₂) which was earlier used for dry-cleaning nowadays liquefied carbon dioxide along with a suitable detergent is used. It will cause less harm to groundwater. Hydrogen peroxide (H₂O₂) is also nowadays used for bleaching clothes which gives better results.

2. Bleaching of paper: Cl₂ gas which was earlier used for bleaching paper-has been replaced by hydrogen peroxide (H₂O₂) with a suitable catalyst.

3. Synthesising chemicals: Acetaldehyde (CH₃CHO) is now prepared by one-step oxidation of ethylene (H₂C = CH₂) in the presence of an anionic catalyst in an aqueous medium with a yield of 90% on a commercial scale.

In a nutshell, Green chemistry is a cost-effective approach, which involves

1. Reduction in cost
2. Reduction in energy consumption
3. Reduction in waste production.

By going through these CBSE Class 11 Chemistry Notes Chapter 13 Hydrocarbons, students can recall all the concepts quickly.

Hydrocarbons Notes Class 11 Chemistry Chapter 13

→ Classification-classification of hydrocarbons.

→ Alkanes-Nomenclature. isomerism, preparation, properties of alkanes, conformations.

→ Alkenes-structure of double bonds, Nomenclature, Isomerism, preparation and properties.

→ Alkynes-Nomenclature of isomerism, the structure of the triple bond, preparation & properties.

→ Aromatic hydrocarbons-Nomenclature & isomerism structure of benzene, Aromaticity, preparation & properties.

→ Directive influence of a functional group in mono-substituted benzene.

→ Carcinogenicity & Toxicity Benzene of polynuclear hydrocarbons.

→ Hydrocarbons: Hydrocarbons are the compounds of carbon & hydrogen only. Hydrocarbons are mainly obtained from coal & petroleum.

→ Petrochemical: Petrochemicals are the prominent starting material used for the manufacture of a large number of commercially important products.

→ L.P.G.: Liquified petroleum gas

→ C.N.G.: Compressed natural gas.

→ Classification of hydrocarbons: Saturated, unsaturated, cyclic (alicyclic) & Aromatic

→ Important reactions of Alkanes: Free radical substitution, combustion, oxidation & aromatization.

→ Alkenes & Alkynes: Undergo mainly addition reactions, (electrophilic additions).

→ Aromatic hydrocarbons: Despite having unsaturation undergo mainly electrophilic substitution reactions

→ Conformation Isomerism: Alkanes show conformational isomerism due to free rotation along with the C – C sigma bonds. Out of staggered of the eclipsed conformations of ethane, staggered conformations are more stable as hydrogen atoms are farthest apart.

→ Geometrical Isomerism: Alkanes exhibits geometrical isomerism (cis-trans) due to restricted rotation around the carbon-carbon double bond

→ Huckel Rule: Benzene of benzenoid compounds show aromatic character. Aromaticity, the property of being aromatic is possessed by compounds having specific electronic structure characterized by Huckel Rule (4n + 2) π electron rule.

→ Carcinogenic property: Some of the polynuclear hydrocarbons having fused benzene ring system have carcinogenic property.

→ Activation & deactivation of benzene ring: The nature of groups or substituents attached to the benzene ring is responsible for activation or deactivation of the benzene ring towards further electrophilic substitution and also for orientation of the incoming group.

Friedel-crafts Reaction:
1. Friedel craft alkylation reaction

2. Friedel craft acylation reaction


Structure of Benzene

Kekule’s Structure

→ Markownikov Rule: The rule states that the negative part of the addendum gets attached to that carbon atom which possesses a lesser number of hydrogen atoms as:
CH₃ – CH = CH₂ + HBr →

2-Bromopropane (Main product)
(ii) CH₃ CH₂ CH₂Br
1-Bromopropane (Minor product)

→ Lindlar’s Catalyst: Partially deactivated palletised charcoal is known as Lindlar’s Catalyst.

Chapter In Brief:
Hydrocarbons are the compounds of carbon and hydrogen only, Alkanes, Alkenes, alkynes, and aromatic compounds constitute hydrocarbons. Alkanes are saturated hydrocarbons containing carbon-carbon single bonds. Alkenes are unsaturated hydrocarbons containing at least one

double bonds, whereas alkynes are unsaturated hydrocarbons containing at least one — C ≡ C — triple bond.

Alkanes: Earlier known as paraffin, the general formula of their homologous series is C_nH_2n+2.

Methane, the first member is having a tetrahedral shape according to VSEPR Theory. It is multiplanar in which a carbon atom lies at the centre and four hydrogen atoms lie at the four corners of a regular tetrahedron. H-C-H bond angle is 109.5°. In alkenes, C-C and C-H bond lengths are 154 pm and 112 pm respectively. C-C and C – H bonds are formed by head-on the overlapping of sp³ hybrid orbitals of carbon and Is atomic orbitals of hydrogen atoms.

Nomenclature & Isomerism in Alkanes:
The first three members of the alkane family namely methane, ethane and propane have only one structure but higher alkanes can have more than one structure.
e.g. C₄H₁₀ have the following two structures


They are called Chain Isomers
Similarly, C₅H₁₂ have the following three structures
1. CH₃-CH₂-CH₂-CH₂-CH₃
: Pentane (n-pentane) b.p. 309 K
2.
: 2-Methyl butane (isopentane) b.p 301K
3.
: Dimethylpropane (neopentane) b.p. 282 K.

1, 2, 3 are the chain isomers of pentane. They differ in their boiling points and other properties, though they have the same molecular formula. This difference in properties is due to the difference in their structures, they are termed Structural Isomers.

Preparation of Alkanes:
Petroleum and natural gas are the main sources of alkanes. However, alkanes can be prepared by the following methods.
1. From unsaturated hydrocarbon by hydrogenation.

2. From alkyl halides:
1. Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes,

2. By Wurtz reaction: Alkyl halides on treatment with sodium in dry ether give higher alkanes. This method is used to prepare higher alkanes containing an even number of carbon atoms.

3. From carboxylic acids
1. Sodium salts of fatty acids on heating with soda-lime [a mixture of NaOH + CaO] give alkanes. The process is called decarboxylation [Removal of a molecule of CO₂]

2. Kolbe’s electrolytic method: An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkanes containing an even number of carbon atoms.

Properties Of Alkanes
(A) Physical Properties:

1. Alkanes are almost non-polar due to the covalent nature of C-C and C—H bonds and due to very little difference of electronegativity between C and H atoms. Therefore, they are insoluble in water but soluble in organic solvents.
2. Due to weak van der Waals forces, the first four members (from C₁ to C₄) are gases. The next thirteen (C₅ to C₁₇) are liquids and those containing 18 carbon atoms or more solids at 298 K.
3. They are colourless and odourless.
4. Their boiling points increase with the increase in molecular mass as shown in the table below.

Table: Variation of melting point and boiling point in alkanes

It is due to fact that intermolecular van der Waals forces increase with the increase in molecular size or surface area of the molecules. For example, among the isomeric pentanes.

B. Pt of n-pentane is highest (309.1 K), whereas that of 2, 2- dimethyl propane is the lowest (282.5 K). With the increase in the number of branched chains, the molecule attains the shape of a sphere. This results in decreased surface area and hence weaker intermolecular van der Waals forces thus lowering the boiling points.

Chemical Properties Of Alkanes
1. Substitution Reaction: Halogenation

Order of reactivity of halogens is F₂ > > Cl₂ > Br₂ > I₂
Rate of replacement of hydrogens of alkanes is: 3° > 2° > 1°
Fluorination is too violent to be controlled.

Bromination is similar. Iodination is very slow and a reversible reaction. It can be carried out in the presence of some oxidising agents like HNO₃ or HIO₃.
CH₄ + I₂ ⇌ CH₃I + HI
HIO₃ + 5HI ⇌ 3I₂ + 3H₂O
Substitution of halogens in alkanes proceeds via a free-radical mechanism.

2. Combustion: Alkanes on heating in the presence of air or oxygen are completely oxidised to carbon dioxide and water with the evolution of a large amount of heat.
CH₄(g) + 2O₂ → CO₂(g) + 2H₂O (l); Δ_cH°=- 890 kJ mol^-1
C₄H₁₀(g) + 6\(\frac{1}{2}\)O₂(g) → 4 CO₂(g) + 5H₂O (1); Δ_cH° = -2876 kJ mol^-1

Due to the evolution of large amount of heat during combustion, alkanes are used as fuels.
During incomplete combustion in insufficient supply of air or Oxygen, carbon black is formed.

3. Controlled Oxidation: In a regulated supply of air or oxygen at high pressure and in the presence of suitable catalysts, alkanes give a variety of oxidation products.

(iv) Ordinarily alkanes resist oxidation but alkanes having tertiary H atoms can be oxidised to corresponding alcohols by KMnO₄.

4. Isomerisation: n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerises to branched-chain alkanes.

5. Aromatisation: n-alkanes having six or more C atoms on heating to 773 K at 10-20 atmospheric pressure in the presence of oxides of V, Mo or Cr supported over alumina gel dehydrogenated and cyclised to benzene and its homologues. This reaction is termed Aromatisation or reforming.

6. Reaction with steam

7. Pyrolysis: Higher alkanes on thermal decomposition give lower alkanes, & a mixture of alkanes. The process is also called Cracking.

Conformations:
Alkanes contain C-C sigma (a) bonds. Free-rotation around C – C bond is possible. Such different Spatial, arrangement of atoms obtained by rotation around the C — C bond is called Conformations or Conformers or Rotamers.

Ethane (C₂H₆) has two major conformational isomers amongst several spatial arrangements differing from each other by a small energy barrier.

One is called eclipsed form which is less stable as it is associated with more energy [due to repulsion of electrons] and the other is called staggered form which is more stable as it is associated with lower energy. Any other intermediate confrontation is called a skew form.

Eclipsed and staggered forms of ethane (C₂H₅) can be represented by Sawhorse and Newman Projections as shown below of all the conformations of ethane.
1. Sawhorse Projections

Sawhorse projections of change

2. Newman’s Projections

Newman projections of ethane

The staggered form has the least torsional strain and the eclipsed form the maximum torsional strain. The energy difference between the two extreme forms is of the order of 12.5 kJ mo^-1 which is very small. These forms have not been separated.

Alkenes. Alkenes are unsaturated hydrocarbons containing at least one

(double bond) or —C = C— (Triple bond)
They are also called Olefins. The general formula of alkenes is C_nH_2n.

Structure of Double Bond
Cabon atoms constituting a double bond undergo sp² hybridisation. The double bond contains one strong sigma (a) bond and one weak Pi (π) bond. The electrons of the π bond are delocalised and is thus a source of electrons. Any electrophile can come and attack it. That is why alkenes undergo electrophilic addition reactions.

The double bond is shorter in bond length (134 pm) than the C-C single bond (154 pm), π bond is a weaker bond due to poor overlapping between the two 2p orbitals. The strength of the double bond (bond enthalpy 681 kJ mol^-1) is greater than that of a C—C single bond (bond enthalpy 348 kJ mol^-1) in ethane.

Orbital picture of ethene depicting bonds only


Original picture of ethene showing formation of (a) π-bond. (b) π-cloud and (c) bond angles and bond lengths

Nomenclature of Alkenes:
In the IUPAC system, the longest chain of carbon atoms containing the double bond is ‘selected’. The numbering of the chain is done from the end which is nearer to the double bond. The suffix ‘ene’ replace ‘ane’ of alkanes.

Put n = 2 in C_n H_2n; C₂H₄ or H₂C = CH₂ is ethylene (common name) and ethene in IUPAC system.

Isomerism in Alkenes
Alkenes show both structural isomerism and geometrical isomerism.
(a) Chain isomerism

(b) Position isomerism
CH₃ – CH₂ – CH = CH₂ But -1-ene
and CH₃ – CH = CH – CH₃ But-2-ene
are position isomers as they differ in the position of the functional group.

(c) Geometrical isomerism
C_xy = C_xy and C_xy type of alkenes show geometrical isomerism
e.g. But-2-ene CH₃ – CH = CH – CH₃ exists in two forms- called geometrical or cis-trans isomers as shown below.

When the identical atoms or groups lie on the same side of the double bond it is called cis-isomer
When the identical atoms or groups lie on the opposite side of the double bond it is called trans-isomer.

The restricted rotation of atoms or groups around the doubly bonded carbon atoms gives rise to different geometries to such compounds. The stereoisomers of this type are called geometrical isomers.

Due to different Spatial arrangements of atoms or groups, geometrical isomers differ in their properties like m.p., b.p., dipole moment, solubility etc.

Cis-form of but-2-ene is more polar than the transform. (Dipole moment) p of cis-form is 0.35 Debye whereas p of transform is almost zero, or trans-2-butene is non-polar. In the transform, two methyl groups being in opposite directions cancel polarities due to each C – CH₃ bond.

In the case of solids, it is found that the trans isomer has higher m.p. than cis form. This is due to the better symmetry of the trans-isomers. Trans solids fit well into the crystal lattice.

Preparation 0f Alkenes
1. From Alkynes: Alkynes on partial reduction with a calculated amount of dihydrogen in the presence of partially deactivated palletised charcoal called Lindlar’s Catalyst to give cis-alkenes. However, alkynes on reduction with sodium in liquid ammonia form trans-alkenes.

2. From Alkyl Halides: Alkyl halides on heating with alcoholic potash (potassium hydroxide dissolved in alcohol) undergo dehydrohalogenation to give alkenes. This is an example of a β-Elimination reaction.

For halogens, the rate of reaction is Iodine > bromine > chlorine while for alkyl groups, it is tert > sec > prim.

3. From vicinal dihalides: Vicinal (on two adjacent C atoms) dihalides on treatment with zinc undergo dehalogenation to give alkenes.

4. From the acidic dehydration of alcohols: Alcohols on heating with conc. H₂SO₄ lose a molecule of H₂O (β-elimination reaction) to form an alkene.

Properties of Alkenes:
Physical properties:

1. The first three members are gases, the next 14 are liquids and the higher ones are solids.
2. Except for ethene, which has a pleasant smell, all alkenes are odourless and colourless.
3. They are insoluble in water but fairly soluble in non-polar solvents like benzene, petroleum, ether etc.
4. They show a regular increase in b.p. with an increase in size [For every — CH₂— group added b.p. increases by 20—30 K] Like alkanes, straight-chain alkenes have higher b.p. than isomeric branched ‘ alkenes.
5. Like alkanes, alkenes are generally non-polar but certain, alkenes are weakly polar due to their unsymmetrical geometry.

Chemical Properties:
(a) Addition reactions:
1. Addition of H₂ (catalytical hydrogenation)

2. Addition of halogens: [Electrophilic addition] Br₂ is a reddish-orange liquid that adds to the unsaturated site to give a colourless product. This reaction is used as a test of unsaturation.

3. Addition of hydrogen halides
The order of reactivity is HI > HBr > HCl
CH₂ = CH₂ + H – Br → CH₃ – CH₂Br
Markovnikov Rule. [Addition of HX to unsymmetric alkenes] “The negative part of addendum (the molecule to be added) goes to that carbon atom of the unsymmetrical alkene which is attached to lesser number of carbon atoms”.

The modern version of Markovnikov Rule. The product is formed from the more stable carbocation.

The more stable carbocation [which predominates because it is former faster] reacts with Br- to form the product.

Anti-Markovnikov Addition or Peroxide/Kharash Effect

In the presence of peroxide, the addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. This happens only with HBr but not with HCl and HI. This addition reaction was observed by M.S. Kharash and F.R. Mayo in 1933 at the University of Chicago. This reaction is known as peroxide or Kharash effect or addition effect or addition reaction anti to Markovnikov rule.

Mechanism: Peroxide effect proceeds via free radical chain mechanism as given below:


The secondary free radical obtained in the above mechanism (iii) is more stable than the primary. This explains the formation of 1 — bromopropane as the major product. It may be noted that the peroxide effect is not observed in addition to HCl and HI.

This may be due to the fact that the H – Cl bond being stronger (430.5 kJ mol^-1) than H — Br bond (363.7 kJ mol^-1), is not cleaved by the free radical, whereas the H – I bond is weaker (296.8 kJ mol^-1) and iodine free radicals combine to form iodine molecules instead of an addition to the double bond.

4. Addition of sulphuric acid. Cold, concentrated sulphuric acid adds to alkenes in accordance with the Markovnikov rule as a result of electrophilic addition to form alkyl hydrogen sulphate.

5. Addition of water. In the presence of a few drops of the cone. H₂SO₄, alkenes undergo hydration with water in accordance with the Markovinkov rule to form alcohols.

6. Oxidation: (a) Alkenes on reaction with cold, dilute, 1 % alkaline potassium permanganate (KMnO₄) solution called Baeyer’s Reagent produce vicinal glycols. The colour of KMnO₄ is discharged, It is also used as a test of unsaturation.

(b) Acidic KMn04 or acidic K₂Cr₂O₇ oxidizes alkenes to ketones and/or acids depending upon the nature of the alkene and the experimental conditions

7. Ozonolysis. It involves the addition of O₃ molecules to the alkene to form ozonide followed by cleavage by Zn/H₂O to form aldehydes and ketones.


8. Polymerisation. When a large number of ethene molecules combine at high temperature, high pressure in the presence of a catalyst, Polythene is obtained.

These polymers are of great use in the manufacture of plastic bags, squeeze bottles, toys, pipes radio and TV cabinets, milk crates, plastic buckets and other moulded articles.

Alkynes: Alkynes are unsaturated hydrocarbons containing at least one — C = C — triple bond.
General formula: C_nH_2n-2
Common & I.U.P.A.C. names of Alkynes
n = 2 C₂H₂ H – C ≡ C – H Acetylene Ethyne

n = 3 C₃H₄ CH₃ — C ≡ CH MethylacetylenePropyne

n = 4 C₄H₆

• CH₃CH₂C = CH Ethylacetylene But-l-yne
• CH₃ – C = C-CH₃ Dimethylacetylene But-2-yne

n = 5 C₅H₈

Structures (i) and (iii) are position isomers
Structures (i) and (ii) and (iii) are chain isomers
Structure of Triple Bond.

Each carbon atom of ethyne has two sp hybridized orbitals. Carbons-carbon sigma (a) bond is obtained by the head-on overlapping of the two sp hybridised orbitals of the two carbon atoms. The remaining sp hybridised orbitals of the two carbon atoms. The remaining sp hybridized orbital of each carbon atom undergoes overlapping along the internuclear axis with the Is orbital of each of the two hydrogen atoms forming two C — H sigma bonds. H – C—C bond angle is 180°.

Each carbon has two unhybridised p orbitals which are perpendicular to each other as well as to the plane of the C – C sigma bond. The 2p orbitals of one carbon atom are parallel to the 2p orbitals of the other carbon atom, which undergo lateral or sideways overlapping to form two pi (p) bonds between two carbon atoms. Thus ethyne molecule consists of one C — C s bond, two C >- Hs bonds and two C — C p bonds.

The strength of the C = C bond (bond enthalpy 823 kJ mol^-1) is more than those of the C = C bond (bond enthalpy 681 kJ mol^-1) and C – C bond (bond enthalpy 48 kJ mol^-1). The C = C bond length is shorter (120 pm) than those of C = C (134 pm) and C – C) (154 pm). The electron cloud between two carbon atoms is cylindrically symmetrical about the internuclear axis. Thus ethyne is a linear molecule.

Orbital picture of ethyne showing (a) sigma overlaps (b) pi overlaps bond angles and bond lengths.

Preparation of Acetylene (Ethyne). Commercially, it is prepared by the action of water on calcium carbide.
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ (Ethyne)

2. From Vicinal Dihalidies. Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation.

Properties of Alkynes:
Physical properties.

1. First, three members are gases, the next eight are liquids and the higher ones are solids.
2. All alkynes are colourless.
3. Except for enthene which has a characteristic odour, others are odourless.
4. Alkynes are weakly polar in nature.
5. They are lighter than water and immiscible with water but soluble in organic solvents like ethers, benzene etc.
6. Their m, p., b, p, and density increases with an increase in molar mass.

Chemical Properties: Alkynes show usual addition reactions, acidic reactions and polymerisation reaction.
A. Acidic character of alkynes: Unlike alkenes, ethyne shows acidic reactions.

Alkanes, alkenes, and alkynes follow the following trend in their acidic behaviour.

1. H – C ≡ C – H > CH₂ = CH₂ > CH₃ – CH₃
2. H – C ≡ C – H > CH₃ – C ≡ CH > > CH₃ – C ≡ C – CH₃

B. Addition reactions,
1. Addition of dihydrogen.
Alkynes contain a triple bond. Therefore, they add up two molecules of H₂.

2. Addition of Halogens. When Br2 is added to alkynes, the reddish-orange colour of Br₂ disappear. It is a test of unsaturation.

3. Addition of hydrogen halides [HCl, HBr, HI]
Two molecules get added to alkynes to form gem dihalides.

4. Addition of water

5. Polymerisation

Aromatic Hydrocarbons or Arenes: Aromatic compounds containing benzene ring are known as Benzenoids and those not containing a benzene ring are called Non-Benzenoids. Some of the arenas are given below.


Where o = ortho (1,2)
m = meta (1, 3)
p = para (1, 4)

Structure of Benzene:

1. Molecular formula C₆H₆ indicates that benzene is an unsaturated hydrocarbon.
2. The unusual stability of benzene and no change of orange-red colour of Br₂ in addition to benzene ruled out the open chain structure of benzene.
3. It forms a triozonide which indicates the presence of three double bonds.
4. Benzene produces one and only one monosubstituted derivative which indicates that all the six-carbon and six hydrogen atoms of benzene are identical.
5. A. Kekule’ in 1865 proposed the cyclic structure for benzene with alternate single and double bonds in carbon atoms with each C atom carrying one hydrogen.

Kekule’ suggested the oscillating nature of double bonds.

Resonance and Stability of Benzene
Benzene is a resonance hybrid of various, resonating structures. The two structures are given above by Kekule’ are the main contributing st; lectures. The hybrid structure is (c) is given below.

The circle represents the six electrons that are delocalised between the six carbon atoms of the benzene ring.

Orbital Picture of Benzene.
All the six carbon atoms of benzene are sp² hybridised. Two of these three sp² hybrid orbitals of each C atom overlap with sp² hybrid orbitals of adjacent C atoms to form six C – C single bonds which are in the hexagonal plane. The remaining sp² orbital of each C atom overlaps with the s-orbital of each hydrogen atom to form six C — H single sigma bonds. Each C atom is now left with one unhybridised p- orbital perpendicular to the plane of the ring as shown on the next page.

The unhybridised p orbital of carbon atoms is close enough to form π (Pi) bond by sidewise overlap. These overlaps can be of overlaps of p-orbitals of C₁ — C₂, C₃ — C₄, C₅ – C₆ or C₃, C₄ — C₅, C₆ – C₁ respectively as shown in the following figures.

(a)

(b)
X-ray diffraction data reveals that benzene is a planar molecule. The six n electrons are delocalised and spread on the whole of the molecule: one half of the electron cloud above and the other half below the plane of the benzene ring. The presence of delocalised n electrons in benzene makes it more stable than the imaginary cyclohexatriene.

or

(Electron cloud)

If there were three single C – C bonds and three alternate C = C bonds present in benzene, the bond lengths should have been 154 pm and 134 pm respectively. In benzene there are neither C – C double bonds present as all the six C — C bonds in benzene are exactly alike and have a bond length of 139 pm. Thus the absence of pure double bonds in benzene accounts for the hesitation on the part of benzene to take part in additional reactions. Due to its extra stability, it prefers to show substitution reactions.

Aromaticity: Benzene is considered a parent aromatic compound. Now the name is applied to all the ring systems whether or not having benzene ring, possessing the following characteristics.

1. It should be planar
2. Complete delocalisation of % electrons in the ring.
3. Presence of (4n + 2) n electrons in the ring where n is an integer (n = 0, 1, 2). This is called Huckel Rule.

Preparation of Benzene
Benzene is commercially isolated from the ‘Light oil fraction’ of coal tar. However, it may be prepared in the laboratory by the following methods.
1. From ethyne

2. Decarboxylation of the aromatic acids Sodium salt of benzoic acid on heating with soda lime gives benzene.

3. Reduction of Phenol in the presence of zinc dust gives benzene

Properties of Benzene (Aromatic hydrocarbons)
Physical properties

1. Aromatic hydrocarbons are non-polar.
2. They are colourless liquids or solids with a characteristic aroma.
3. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents.
4. They burn with a sooty flame.

Chemical Properties
Arenes undergo electrophilic substitution reactions. However, under special conditions, they undergo addition and oxidation reactions.

Electrophilic Substitution Reactions of arenes are nitration, halogenations, sulphonation, Friedel Craft’s reactions.
In all these reactions, the attacking reagent is an electrophile E®.
1. Nitration.

2. Halogenation: Arenes react with halogen in the presence of Lewis acids like FeCl₃, FeBr₃, or AlCl₃ to yield halo arenes.

Order of reactivity of halogens is Cl₂ > Br₂ > I₂.

3. Sulphonation: Here H of the benzene ring is replaced by sulphonic group (— SO₂ OH). It is carried out by heating benzene with fuming sulphuric acid (oleum).

4. Friedel Craft’s Reaction:
(a) Alkylation: On reacting benzene with an alkyl halide in the presence of anhydrous Aluminium chloride, alkyl benzene is formed.

(B) Acylation: On treating benzene with an acyl chloride in the presence of Lewis acids (AlCl₃) gives acyl benzene.

Mechanism of electrophilic substitution reactions: It involves three steps:

1. Generation of an electrophile.
2. Formation of a resonance-stabilised carbocation intermediate.
3. Removal of proton H⁺ to form the product.

1. Generation of Eelecrophile (E⁺): In the above reactions electrophiles like Cl⁺ (chloronium ion) is generated during chlorination by reacting with any. AlCl₃.

In the case of Nitration, NO₂ (nitronium ion) is generated.

2. Formation of carbocation (arenium ion) results with one of the carbon getting sp³ hybridised on the attack of the electrophile (E)⁺

Sigma complex (arenium ion)
The intermediate arenium ion gets stabilised by resonance.

3. Removal of a proton (H⁺)


2. Addition reactions: Under drastic conditions of high temperature and or pressure in the presence in the presence of nickel catalyst, dihydrogen gets added to the benzene.

In the presence of ultraviolet light, three molecules of Cl₂ get added to benzene to form Benzene hexachloride [BHC] C₆H₆C₁₆ also called Gammaxene.

3. Oxidation by combustion: When heated in air, benzene burns with a sooty flame producing CO₂ and H₂O.
C₆H₆ + O₂ → 6 CO₂ + 3H₂O
General combustion reaction for any hydrocarbon is
C_xH_y +(x + y/4)O₂ → xCO₂ + y/2H₂O.

Directive Influence of a Functional Group in Monosubstituted Benzene
Ortho and para directing groups: The groups which direct the incoming group to ortho & para positions are called ortho & para directing groups. In phenol, for example, — OH (hydroxy) group attacked to benzene directs the new (or coming group) to ortho para positions as explained below:

From the above structures, it is clear that electron density is more at ortho & para positions (structure II, III & IV) to the – OH group. Hence the coming electrophile will prefer to attack ortho & para position rather than meta. However due to the — I effect exerted by the — OH group, electron density at o—&p—position is slightly reduced. But overall, there is an increase of electron density ato-Scp- position. Hence the substituent at o—&p — positions to the -OH group.

Therefore, the -OH group is an activating group, as it activates the benzene ring for the attack of an electrophile. Other activating groups are NH₂, -NHR, NHCOCH₃, -OCH₃, -CH₃, -C₂H₅ etc.

Halogens are a class among themselves. They are deactivating and at the same time o—Scp — directing. Because of the, I effect, the overall electron density on benzene decreases. It makes further substitution difficult. However, due to resonance, the electron density on the o—& p — position is greater than at the meta position. Hence they are also o— & p — directing.

Meta-directing groups. The groups which when present in the benzene ring direct the incoming groups to meta position are called meta-directing groups. Some of the meta-directing groups are

(nitro group), for example, reduces the electron density in the benzene ring due to its — I effect. Nitrobenzene is a resonance hybrid of the following five canonical structures.

In this case, the electron density on the benzene ring decreases making further substitution difficult. Therefore these groups are called deactivating groups. The electron density on the o – and p – position is comparatively less than that at the meta position. Hence, the electrophile attacks on comparatively electron-rich meta position, resulting in meta-substitution.

Carcinogenicity and Toxicity:
Benzene and polynuclear hydrocarbons containing more than two fused benzene rings are toxic and said to possess cancer-producing (carcinogenic) property. They enter into the human body and undergo various biochemical reactions and finally damage DNA and cause cancer. Some of the carcinogenic hydrocarbons are given below. Such polynuclear hydrocarbons are formed on incomplete combustion of organic materials like tobacco coal and petroleum.

By going through these CBSE Class 11 Chemistry Notes Chapter 12 Organic Chemistry Some Basic Principles and Techniques, students can recall all the concepts quickly.

Organic Chemistry Some Basic Principles and Techniques Notes Class 11 Chemistry Chapter 12

→ Carbon: Tetra valency of carbon, shape of organic compounds & characteristic features of π-bond.

→ Structural representation of organic compounds: Complete, condensed & bond line structural formulae.

→ A 3-dimensional representation of organic molecules & classification of organic compounds.

→ Acyclic or open chain compounds & Alicyclic or closed chain compounds or ring compounds & functional groups.

→ Homologous series, Nomenclature of organic compounds & I.U.P.A.C. nomenclature of alkanes.

→ Nomenclature of organic compounds having a functional group or groups & nomenclature of substituted benzene compounds.

→ Isomerism: Structural, chain, position, functional group isomerism, metamerism & stereoisomerism.

→ Fundamental concepts in organic reaction mechanism: Fission of a covalent bond, Nucleophiles & Electrophiles, Electron movement in organic reactions. Electron displacement effects in covalent bonds. Inductive effect, resonance structure & resonance effect.

→ Electromeric effect (E-effect), Hyperconjugation, types of organic reactions & mechanisms.

→ Methods of purification of organic compounds: Sublimation, crystallization, distillation, differential extraction, chromatography.

→ Qualitative analysis of organic compounds detection of C & H, N, S, halogens & for PO₄^3-.

→ Quantitative analysis of organic compounds: Elemental detection in the form of a percentage, C, H, N-(Dumas method, Kjeldahl’s method), halogens, sulfur, phosphorus & oxygen.

→ Orbital hybridization concept: The nature of the covalent bonding in organic compounds can be described in terms of the orbitals hybridization concept.

→ Three-dimensional representation of organic compounds: Three-dimensional representation of organic compounds on paper can be drawn by wedge & dash formula.

→ Functional group: A functional group is an atom or group of atoms bonded together in a unique fashion & which determines the physical & chemical properties of compounds.

→ I.U.P.A.C.: International union of pure & applied chemistry.

→ Organic reaction mechanism: Organic reaction mechanism concepts are based on the structure of the substrate molecule. Fission of a covalent bond, the attacking reagents, the electron displacement effects & the conditions of the reaction.

→ Cleavage of covalent bond: A covalent bond may be cleaved in a heterolytic or homolytic fashion. A Heterolytic cleavage yields carbocations or carbanions & a homolytic cleavage gives free radicals as reactive intermediates.

→ Nucleophile & Electrophile:

• Nucleophile – Electron pair donor
• Electrophile – Electron pair acceptor.

→ Organic reactions:

1. Substitution reactions
2. Addition reactions
3. Elimination reactions
4. Re-arrangement reactions

→ Methods of purifications of organic compounds:

1. Sublimation
2. Distillation &
3. Differential extraction

→ Chromatography is a useful technique of separation, identification & purification of compounds. It is classified into two categories adsorption & partition chromatography. Lassaigne’s Test: N, S, halogens & phosphorus are detected by Lassaigne’s test.

→ Estimation of C & H: Carbon & hydrogen are estimated by determining the amounts of CO₂ & water produced. Estimation of Nitrogen: Nitrogen is estimated by Duma’s or Kjeldahl’s method.

→ Halogens Estimation: Halogens are estimated by various methods. Estimation of S & Phosphorus: S & P are estimated by oxidizing them to sulphuric & phosphoric acid respectively.

→ The percentage of oxygen: The percentage of oxygen is usually determined by subtracted (the sum of percentages of all other elements present in the compound) out of 100.

→ Retardation factor:
Rf = \(\frac{\text { Distance moved by the substance from base line }}{\text { Distance moved by solvent from base line }}\)
(a) Percentage of carbon = \(\frac{12 \times m_{1} \times 100}{44 \times m}\)
m₁ = mass of CO₂
m = mass of organic compound

(b) Percentage of Hydrogen = \(\frac{2 \times m_{1} \times 100}{18 \times m}\)
m₁ = mass of H₂O
m = mass of organic compound

(c) Percentage of nitrogen by Dumas method = \(\frac{28 \times V \times 100}{22400 \times m}\)
V = Volume of nitrogen m mass of organic compound

(d) Percentage of nitrogen by kJeldahl’s method = \(\frac{1.4 \times \mathrm{M} \times 2\left(\mathrm{~V}-\mathrm{V}_{1} / 2\right)}{m}\)
m mass of organic compound
M = Molarity of H₂SO₄ taken
V = Volume of H₂SO₄ of molarity-M
V₁ = Volume of NaOH of molarity-M used for titration of excess of H2S04

(e) Percentage of halogens = \(\frac{\text { Atomic mass of }(\mathrm{X}) X m_{1} g \times 100}{\text { molecular mass of }(\mathrm{AgX}) X m}\)
m = mass of organic compound
m₁ = mass of AgX formed
X = halogen atom

(f) Percentage of sulphur = \(\frac{32 \times m_{1} \times 100}{233 \times m}\)
m = mass of organic compound
m₁ = mass of BaSO₄ formed

(g) Percentage of Phosphorus:
If Phosphorus is estimated as Mg₂P₂O₇
= \(\frac{62 \times m_{1} \times 100}{222 \times m}\)
m = mass of organic compound
m₁ = mass of Mg₂P₂O₇
222 = molar mass of Mg₂P₂O₇

If Phosphorus is estimated as (NH₄)₃ PO₄.12MoO₃ then percentage of Phosphorus = \(\frac{31 \times m_{1} \times 100}{1877 \times m}\)
here m₁ = mass of (NH₄)₃ PO₄.12MoO₃
1877 = molar mass of (NH₄)₃PO₄.12MoO₃

(h) Percentage of Oxygen:
= \(\frac{32 \times m_{1} \times 100}{44 \times m}\)
m = mass of organic compound
m₁ = mass of carbondioxide

Chapter In Brief:
Berzelius, A Swedish chemist proposed that a Vital Force was responsible for the formation of organic compounds. F. Wohler gave a death blow to the Vital Force theory when he synthesized organic compound urea from an inorganic compound ammonium cyanate.

Tetravalence of Carbon: Shapes of organic compounds: The formation of CH₄, C₂H₆ is due to sp³ hybridization of C; formation of CH₄ is on the basis of sp² hybridization of C, and formation of C₂H₂ is on the basis of sp hybridization of C. The presence of double bond in H₂C=CH₂ and triple bond in HC = CH is due to the presence of one π and two π bonds respectively in them.

In H₂C=CH₂, rotation about C-C bond is hindered due to the presence of π bond between the two C atoms, sp³ hybridization gives rise to tetrahedral shape t

o CH₄, sp² hybridization gives rise to a trigonal planar arrangement to C₂H_4, and sp hybridization gives linear shape to C₂H₂. An sp³ hybrid orbital can overlap with Is orbital of hydrogen to give a C—H bond (sigma a single bond). Overlap of an sp² orbital of one carbon with an sp² orbital of another results in the formation of a carbon-carbon bond.

The unhybridized p-orbitals on two adjacent carbons can undergo lateral (side-by-side) overlap to give a pi (π) bond. Organic compounds can be represented by various structural formulas. The three-dimensional representation of organic compounds on paper can be drawn by the wedge and dash formula.

Organic compounds can be classified on the basis of their structure or the functional groups they contain. A functional group is an atom or group of atoms bonded together in a unique fashion which determines the physical and chemical properties of the compounds. The naming of the organic compounds is carried out by following a set of rules laid down by the International Union of Pure and Applied Chemistry (IUPAC). In IUPAC nomenclature, the names are correlated with the structure in such a way that the reader can deduce the structure from the name.

Structural Representations Of Organic Compounds:
Complete, condensed, and Bond-line structural formulae

stand for the complete structural formulae of ethane, ethene ethyne, and methanol whereas CH₃—CH3₃ (or C₂H₆), H₂C=CH₂ (or C₂H₄), HC ≡ CH (or C₂H₂), and CH₃OH stand for their condensed structural formulae respectively.

Bond-line structural representation of 1,3 butadiene is

and that of 3-methyl octane is

(its condensed formula is CH₃CH₂CH(CH₃) (CH₂)₄CH₃

The bond-line structure of chlorocyclohexane is

Three Dimensional Representation Of Organic Molecules:
The three-dimensional (3-D) structure of organic molecules can be represented on paper by using certain conventions.

For example by using solid

and dashed

wedge formula, the 3-D image of a molecule from a two-dimensional picture can be perceived. The solid wedge projects towards the observer and the dashed wedge projects away from the observer. The bonds lying in the plane of the paper are depicted by using a normal line (—)

The 3-D representation of CH₄ is

Classification of Organic Compounds:

1. Acyclic or Open Chain Compounds: These compounds are also called aliphatic compounds and consist of straight or branched chain compounds.


is acetaldehyde and

is acetic acid.

2. Alicyclic or Closed Chain or Ring Compounds:
Some of the examples of alicyclic /closed chain or ring compounds are as follows:

Aromatic Compounds: Benzenoid Aromatic Compounds:

Non-Benzenoid Compounds

Hetero Cyclic Aromatic Compounds

→ Functional Group:
The functional group may be defined as an atom or group of atoms joined in a specific manner that is responsible for the characteristic chemical properties of the organic compounds. The examples are hydroxyl group (-OH), aldehyde group (-CHO) and carboxylic acid group (—COOH), etc.

→ Homologous Series:
A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the member of the series are called homologs. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in the molecular formula by a — CH₂ unit. There are a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, alkyl halides, alkanols, alkanols, alkenones, alkanoic acids, amines, etc.

e.g. The general formula of alkanols is C_nH_2n+1-OH. Individual members of a homologous series are called Homologues.
CH₃OH, C₂H₅OH, C₃H₇OH are homologs of the alkanol family.

→ Nomenclature of Organic Compounds:
Earlier organic compounds were known by their common or trivial names. For example, HCOOH was called formic acid, CH₃ CHO was called acetaldehyde, and so on.

→ The I.U.P.A.C System Of Nomenclature:
To systematize the naming of millions of organic compounds IUPAC (International Union Of Pure And Applied Chemistry) pattern of naming is adopted.

The I.U.P.A.C. System Of Nomenclature


Common or Trivial names of some organic compounds

Alkyl, Radicals (R) C_nH_2n+1

Table: Some functional Groups and classes of organic compounds:


Note: Students are advised to follow different rules and conventions as per the IUPAC system as given in the Textbook. In the case of polyfunctional compounds, one of the functional groups is chosen as the principal functional group and the compound is named on that basis.

The remaining functional groups which are subordinate functional groups are named as substituents using the appropriate prefixes. The choice of the principal functional group is made on the basis of the order of preference. The order of decreasing priority for the same functional groups is:
-COOH, -SO₃H, -COOR (R = alkyl group)
-COCl, -CONH₂, -C ≡ N, -CHO, > C = O, -OH, -NH₂,

The R, C₆H₅, halogens (F, Cl, Br, I), NO₂, alkoxy (OR), etc. are always prefixed substituents.

For example:
(i) HOCH₂(CH₂)₃CH₂COCH₃ will be named as 7 hydroxyheptan- 2-one
(ii) Br CH₂CH = CH₂ is named as 3-Bromoprop-l-ene.
(iii) CH₂ = CH-CH = CH₂ is Buta-1, 3-diene.

Problem:
Derive the structure of
1. 2-chioropentane
Answer:

2. Pent-4-en-2-ol
Answer:

3. 3-Nitrocyclohexene
Answer:

4. Cyclohex-2-en-l-ol
Answer:

5. 6-Hydroxyheptanal
Answer:

Nomenclature of Substituted Benzene Compounds:




Problem: Write the structural formula of
(a) o-Ethyl anisole
Answer:

(b) p-Nitroaniline
Answer:

(c) 2, 3-dibromo-l-phenyl pentane
Answer:

(d) 4-Ethyl-l-fluoro-2-nitrobenzene
Answer:

Isomerism: The phenomenon of the existence of two or more compounds possessing the same molecular

formula but different properties is known as isomerism. Such compounds are called isomers. The above flow chart shows different types of isomerism.

Types of structural isomerism:
1. Chain isomerism: This type of isomerism is due to the difference in the nature of the carbon chain (i.e., straight or branched) which forms the nucleus of the molecule, e.g.,

2. Position isomerism: It is due to the difference in the position of the substituent atom or group or an unsaturated linkage in the same carbon chain. Examples are


3. Functional isomerism: Two or more compounds having the same molecular formula but different functional groups are called functional isomers and this phenomenon is termed functional group isomerism. For example, the molecular formula C₃H₆O represents an aldehyde and a ketone.

and C₃H₆O represents an ether and alcohol.

4. Metamerism: It is due to the difference in nature of the alkyl group attached to the same functional group. This type of isomerism is shown by compounds of the same homologous series.
For example.

II. Stereoisomers: Stereoisomers are compounds that have the same constitution and sequence of covalent bonds but differ in the relative positions of their atoms or groups in space.

5. Geometrical isomerism: The isomers which possess the same structural formula but differ in the spatial arrangement of the groups around the double bond are known as geometrical isomers and the phenomenon is known as geometrical isomerism. This Isomerism is shown by alkenes or their derivatives. When the similar groups lie on the same side, it is the cis-isomer, while when the similar groups lie on opposite sides, the isomer is trans. For example

Fundamental Concepts In Organic Reaction Mechanism:
In an organic reaction, the organic molecules (substrate) reacts with an appropriate attacking reagent and leads to the formation of one or more intermediates and finally product (s)

The general reaction is depicted as follows:

The substrate is that reactant which supplies carbon to the new bond and the other reactant is called reagent. A sequential account of each step, details of electron movement, energetics during bond breaking and bond formation, and the details of timing, when a reactant is transformed into the product are referred to as Reaction Mechanism.

Fission of a Covalent Bond: It occurs in two ways.
(A) Homolytical Fission/Cleavage or Homolysis
In such fission, each atom gets one electron of the shared pair of electrons.

Alkyl radicals are classified as primary secondary or tertiary. Alkyl radical stability increases as we proceed from primary to tertiary. Organic reactions, which proceed by homolytic fission are called free radical or homopolar, or non-polar reactions.

→ Heterolytical Fission/Cleavage or Heterolysis: The covalent bond breaks in such a way that the shared pair of electrons remains with one of the fragments

The species that has a sextet at the carbon and is positively charged is called a Carbocation (or carbonium ion)
The shape of methyl carbocation C is sp2 hybridized and its shape is Trigonal Planar

The observed order of carbocation stability is

The fission can occur, the other way.

: -CH₃ is called a Carbanion. Such a carbon species carrying a negative charge is called a Carbanion. Their stability decreases as follows:

Nucleophiles & Electrophiles:
(A) Nucleophile (Nu:): A reagent that is electron-rich and is in search of a relatively positive center is called a nucleophile. Example of nucleophiles are

Negatively charged reagents:

(B) Electrophiles: A recent which is electron-deficient and is in search of electron-rick site is called an electrophile Positively charged electrophiles are: H⁺, H₃O⁺, NO₂⁺, R⁺, Br₄
Neutral particles: BF₃, AlCl₃, SO₃

Inductive Effect (I effect):
It is the process of displacement of electrons along the chain of carbon atoms due to the presence of a polar covalent bond at one end of the chain. This is a permanent effect. It is of two types:
(A) -I effect: When the atom or group of atoms of the polar covalent bond is more electronegative than C, it is said to show the -I effect.

It is practically over after C₂
The —I effect of some of the atoms or groups of atoms in decreasing order is
-NO₂ > -CN > -COOH > -F > -Cl > -Br > -I

(B) + I effect: If the substituent attached to the end of the carbon chain is electron-donating, the effect is called + I effect.

The + I effect of some of the atoms or groups of atoms in the decreasing order is

→ Electromeric Effect (E-effect): It involves the complete- transfer of electrons of multiple bonds (double or triple bond) to one of the bonded atoms (usually more electronegative) at the call of the attacking reagent. It vanishes the moment the attacking reagent is removed. It is a temporary effect.

It is also of two types – E and + E effect.
If the electrons of the bond are transferred to that atom of the double bond to which the reagent finally gets attached the effect is called the + E effect.

If the electrons of the double bond are transferred to an atom of the double bond other than the one to which the reagent gets finally attached, the effect is called the — E effect.

→ Resonance Or Mesomerism: The phenomenon of resonance is said to occur whenever for a molecule we can write two or more Lewis structures that differ in the positions of electrons but not in the relative position of atoms. The various Lewis structures are called responding/canonical/contributing structures. The actual structure of the molecule is not represented by any of the resonance structures but is a resonance hybrid of all these canonical structures.

The various resonance structures are separated by a double-headed arrow ↔ Benzene is a resonance hybrid of the two Kekule structures.

Any of the two structures cannot explain all the properties of benzene. But the resonance hybrid which cannot be drawn on the paper and which is the actual structure of benzene will explain all the properties of benzene. For example, there are 3 double bonds and 3 single bonds (3 C = C and 3 C — C) in benzene corresponding to bond lengths of 1.34 Å and 1.54 Å respectively.

But as X-ray diffraction studies point out there are no single or double bonds in benzene and all the C—C bonds are having a bond length of 1.39 Å and are exactly equivalent. The resonance hybrid of benzene is generally shown by III.

Another example of resonance is provided by CH₃NO₂ (nitromethane).

Hyper Conjugation:
It is regarded as no bond resonance. Hyperconjugation is a general stabilizing interaction. It involves delocalization of an electron of C-H bond of an alkyl group directly attached to an atom of the unsaturated system; or to an atom with an unshared p orbital. The electrons of C—H a bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p orbital. Hyperconjugation is a permanent effect.

To understand the hyperconjugation effect, let us take an example of CH₃⁺CH₂ (ethyl cation) in which the positively charged carbon atom has an empty n orbital. One of the C-H CT bonds of the methyl group can align in the plane of this empty n orbital and the electrons constituting the C—H bond in-plane with this π orbital can then be delocalized into the empty π orbital as depicted in Fig.

Orbital diagram showing hyperconjugation in ethyl cation

This type of overlap stabilizes the carbocation because electron density from the adjacent bond helps in dispersing the positive charge. In general, the greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilization of the cation. Thus, we have the following relative stability of carbocations:

Hyperconjugation is also possible in alkenes and alkyl arenes. Delocalisation of electrons by hyperconjugation in the case of an alkene can be depicted as in Fig.(b)

Orbital diagram showing hyperconjugation in propene

There are various ways of looking at the hyperconjugation effect. One of the ways is to regard the C-H bond as possessing partial ionic character due to resonance.

Problem: Explain why (CH₃)₃C⁺ is more stable than CH₃ CH₂⁺ and CH₃⁺ is the least stable cation.
Answer: Hyperconjugation interaction in (CH₃)₃C⁺ is greater than in CH₃CH₂⁺ as the (CH₃)₃C⁺ has nine C—H bonds. In CH₃, vacant n orbital is perpendicular to the plane in which C-H bonds lie, hence cannot overlap with it. Thus CH₃⁺ lacks hyper conjugative stability.

Types Of Organic Reactions:
1. Substitution Reactions

2. Addition reactions
H₂C = CH₂ + HBr → CH₃ – CH₂Br

3. Elimination reactions
CH₃—CHBr—CH₃ + KOH → CH₃-CH = CH₂ + KBr + H₂O

4. Rearrangement Reactions

Methods Of Purification Of Organic Compounds:
The common techniques used for the purification of organic compounds are based on their nature and the impurity present in them. The methods are as follows.

1. Sublimation
2. Crystallization
3. Distillation
4. Differential extraction
5. Chromatography

Finally, the purity of a compound is ascertained by determining its melting point or boiling point. Most of the pure compounds have sharp melting points and boiling points.

1. Sublimation: Some solid substances like camphor, naphthalene, etc. on heating change from solid to vapor state without passing through the liquid phase. The purification technique based on the above principle is known as sublimation and is used to separate sublimable compounds like benzoic acid from non-sublimable compounds like sodium chloride.

2. Crystallisation: It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The impure compound is dissolved in a solvent in which it is sparingly soluble at room temperature but appreciably soluble at a higher temperature. The solution is concentrated by heating to get a nearly saturated solution. On cooling, crystals of the pure substance are removed by filtration.

3. Distillation: The process of distillation is carried out to separate

• volatile liquids from non-volatile impurities and
• liquids having sufficient differences in their boiling points.

Liquids having different boiling points vaporize at different temperatures. The vapors are cooled and get condensed into liquids. They are collected separately. CHCl₃ (b.p. 334 K) and aniline (b.p. 457 K) are easily separated by this method.

Fractional Distillation:
It is resorted to when the difference in boiling points of two liquids is not much. It is carried out through an a.fractionating column fitted over the mouth of the round bottom flask.

One of the technological applications of fractional distillation is to separate different fractions of crude oil in the petroleum industry.

Fractional. Distillation

The vapors of the less volatile liquid condense into the liquid which returns to the flask. The more volatile fraction passes over to the other side, condenses in the water condenser, and is collected in the receiver. When one fraction is completely separated the temperature is raised and the receiver is changed. Now, the second less volatile fraction distills over. Thus the more volatile liquid distills afterward. This is highly successful if the difference in b.p. of two liquids is less than 10-15 K.

Fractional. Distillation

Distillation Under Reduced Pressure:
This method is applicable to purify liquids having very high boiling points and those, which decompose at or below their boiling points. Such liquid is are made to boil at a temperature lower than their normal boiling points by reducing the pressure on their surface. A liquid boils at a temperature at which its vapor pressure becomes equal to the external pressure. The flowsheet diagram for distillation under reduced pressure is shown below.

Distillation under reduced pressure. A liquid boils at a temperature below its vapor pressure by reducing the pressure.

Steam Distillation:
This technique is applied to separate substances, which are steam, volatile, and are immiscible with water. In steam distillation, the steam generator is passed through a heated flask containing the liquid to be distilled. The mixture of steam and the volatile liquid is condensed and collected.

In steam distillation the liquid boils when the sum of the vapor pressures due to the organic liquid (p₁) and that due to water (p₂) become equal to the atmospheric pressure (p) i.e., p = p₁ + p₂ since p1 is lower than p, the organic liquid vapourised at a lower temperature than its b. pt. Thus if one of the substances in water and the other a water-insoluble substance such a mixture will boil close to but below 373 K. Aniline is separated from the aniline-water mixture by this method.

Steam distillation. Steam volatile component volatilizes, the vapors condense in the condenser and the liquid collects in a conical flask.

Differential Extraction:
When an organic compound is present in an aqueous medium, it is separated by shaking with an organic solvent in which is more soluble than water. The organic solvent and the aqueous solution should be immiscible with each other so that they form two distinct layers which can be separated by the separatory funnel. The organic solvent is later removed by distillation or by evaporation to get back the compound.

(a) Differential extraction. Extraction of the compound takes place based on the difference in solubility

Chromatography:
Chromatography is an important technique extensively used to separate mixtures into their components, purify compounds, and also test the purity of compounds. In this technique, the mixture of substances is applied into a stationary phase, which may be a solid or a liquid.

A pure solvent, a mixture of solvents, or a gas is allowed to move slowly over the stationary phase. The components of the mixture get gradually separated from one another. The moving phase is called the mobile phase.

Based on the principle involved, chromatography is classified into different categories. Two of these are:
(a) Adsorption chromatography and
(b) Partition chromatography,

(a) Adsorption Chromatography: Adsorption chromatography is based on the fact that different compounds are adsorbed on an adsorbent to different degrees. Commonly used adsorbents are silica gel and alumina. When a mobile phase is allowed to move over a stationary phase (adsorbent), the components of the mixture move by varying distances over the stationary phase.

Following are two main types of chromatography techniques based on the principle of differentials adsorption.
(a) Column chromatography and
(b) Thin layer chromatography

(a) Column Chromatography: Column chromatography involves the separation of a mixture over a column of adsorbent (stationary phase) packed in a glass tube. The mixture adsorbed on the adsorbent is placed on top of the adsorbent in the column. An appropriate element which is a liquid or a mixture of liquid is allowed to flow down the column slowly. Depending upon the degree to which the compounds are adsorbed, complete separation takes place. The most readily adsorbed substances are retained near the top and the others come down to various distances in the column.

Column chromatography. Different stages of separation of components of a mixture

(b) Thin Layer Chromatography (TLC): Another type of adsorption chromatography, which involves the separation, of substances of a mixture over a thin layer of an adsorbent coated on a glass plate. A thin layer of an adsorbent (silica or alumina SiO₂ or Al₂O₃ gel) is spread over a glass plate. The solution of the mixture to be separated is applied as a small spot about 2 cm above one end of the TLC plate. The glass plate is then placed in a closed jar containing the eluent (see fig. below).

As the eluent rises up the plate the components of the mixture move up along with the eluent to different distances depending on their degree of adsorption and separation takes place. The relative adsorption of each component is expressed in terms of its Retention Factor, i.e., R_f Value
R_f = \(\frac{\text { Distance moved by the substance from baseline }(x)}{\text { Distance moved by the solvent from baseline }(y)}\)

(a) Thin layer chromatography,
(b) Developed chromatogram. Chromatogram being developed

The spots of colored compounds are visible on the TLC plate due to their original color. Fig. (b) on the previous page.

Partition Chromatography:
Paper chromatography is a type of partition chromatography. Water trapped in chromatography paper acts as a stationary phase. It is based on the principle of continuous differential partitioning of components of a mixture between stationary and mobile phases. A strip of paper spotted at the base with the solution of the mixture is suspended in a suitable solvent.

The solvent acts as the mobile phase rise up due to capillary action and flows over the spot. The paper selectively retains different components according to their differing partition in two phases. The paper strip so developed is called Chromatogram. The spots of the separated colored compounds are visible at different heights from the position of the initial spot on the chromatogram. The spots may be observed under U. V. light as discussed in TLC.

Paper chromatography. Chromatography paper in two different shapes.

Qualitative Analysis Of Organic Compounds Detection Of Elements:
The elements present in the organic compound can be detected as follows:
1. Carbon and hydrogen: The given organic compound is mixed with about double the amount of pure and dry copper oxide. The mixture is heated in a hard glass tube. The CO₂ and H₂O produced due to combustion are tested by lime water and anhydrous copper sulfate. The lime water will turn milky and copper sulfate will turn blue.


2. Nitrogen, can be detected as
1. Soda-lime test: When the organic compound is heated with soda lime in a test tube, the evolution of ammonia indicates nitrogen.

2. Lassaigne’s test: A small piece of dry sodium is heated gently in a fusion tube till it melts to a shining globule. Then a small amount of organic substance is added and the tube is heated to red hot. The red hot tube is plunged into distilled water contained in a china dish. The contents of the dish are boiled, cooled, and filtered. The filtrate is known as sodium extract or Lassaigne’s extract.

For the nitrogen test, the sodium extract is made alkaline with a few drops of dil. NaOH. Freshly prepared FeSO₄ solution is added and the contents are warmed. Then a few drops of FeCl₃ are added followed by acidification with cone. HCl or H₂SO₄. The appearance of bluish-green coloration indicates nitrogen.

If the organic compound contains N and S together, sodium thiocyanate (Na CNS) may be formed with the sodium extract which gives blood-red coloration due to the formation of Fe(CNS)₃,

Sulfur: To the sodium extract.
1. Add lead acetate: The formation of black precipitate confirms sulfur.

2. To the other part of sodium extract add a few drops of sodium nitroprusside solution. The appearance of purple color indicates sulfur.

Halogens:
This test can also be done with sodium extract. The extract is boiled with a cone. HNO₃ to expel the gases. It is then cooled and treated with silver nitrate solution. The formation of different colored precipitates confirms halogens.

Quantitative Analysis:
1. Estimation of Carbon and Hydrogen: A known weight of the organic compound is heated with dry cupric oxide in the dry atmosphere free from CO₂. The carbon and hydrogen present, in the organic compound, are oxidized to CO₂ and water. The CO₂ is absorbed in potash bulbs and water is absorbed in CaCl₂ tubes. From the weights of CO₂, and H₂O form, the percentage of C and H are calculated as:

2. Estimation Of Nitrogen: Nitrogen can be estimated by one of the following two methods.
1. Duma’s Method: A known weight of the given organic compound is heated with dry cupric oxide in a current of CO₂. The N0 gas obtained is connected in Scliffs nitrometer at the prevailing temperature and pressure. Then, this volume of N, gas so collected is converted to volume at STP/NTP by using gas equation
P₁V₁/T₁ = P₂V₂/T₂
knowing 22.4 L of N₂ gas at STP weight = 28.0 gm.

Weight and percentage of Nitrogen can be calculated
% of N = \(\frac{28}{22400} \times \frac{\text { Volume of } \mathrm{N} \text { at } \mathrm{STP} \text { in } \mathrm{mL}}{\text { Mass of compound }}\) × 100

2. Kjeldahl’s Method: This is a more convenient method for the estimation of N particularly in foods, fertilizers, drugs etc. This method is, however not applicable to compounds containing nitrogen in the ring (Pyridine, quinoline, etc.) and compounds containing N directly linked to an oxygen atom (eg. NO₂) or another N atom. e.g. A Z O (—N = N—) compounds.

In this method, the given organic compound is treated with a cone. H₂SO₄ to couvert N into (NH₄)₂ SO₄ the ammonium sulfate [(NH₄)₂SO₄ is treated with 40% NaOH solution and the ammonia evolved is neutralized with an excess of a standard acid [known volume V of the acid taken. The excess of the residual acid is titrated with a standard solution of the alkali and the volume of the acid left unneutralized by ammonia (v ml) is noted.

∴ Volume of the acid neutraL ised by ammonia = (V – u) ml.
∴ % of N = \(\frac{1.4 \times N(V+v)}{W}\)
where N = Normality of acid taken
W = wt. of the organic compound.

Estimation Of Halogens: The given organic compound containing halogens is treated in Carius Method with fuming nitric acid in a long-necked Carius Tube and silver nitrate. The halogen present is converted into silver halide. From the weights of silver halide formed and the known weight of the organic compound taken, the percentage of halogen can be calculated.

% of halogen = \(\frac{\text { Atomic mass of halogen }}{108+\text { At. mass of halogen }}\) × \(\frac{\text { Mass of silver halide }}{\text { Mass of substance }}\) × 100

Estimation of Sulphur:
In the Carius method organic compound is treated with fuming HNO₃ and S is precipitated as BaSO₄ by the addition of BaCl₂. From the wt. of BaS04 formed, the percentage of S can be calculated.
% of sulphur = \(\frac{32}{233} \times \frac{\text { Mass of } \mathrm{BaSO}_{4}}{\text { Mass of substance }}\) × 100

Estimation of Phosphorus:
In Carius method, P is quantitatively oxidized to H₃PO₄ by fuming HNO₃ which is precipitated to Mg₂P₂O₇. knowing the wt. of Mg₂P₂O₇ and that of the organic compound, percentage of P can be determined
% of P = \(\frac{62}{222} \times \frac{\text { Mass of } \mathrm{Mg}_{2} \mathrm{P}_{2} \mathrm{O}_{7} \text { formed }}{\text { Mass of the substance }}\) × 100

Estimation of Oxygen:
There is no direct method available to estimate oxygen in the organic compound. The percentage of oxygen is usually found by subtracting the sum of the percentages of all elements present in the compound from 100.

By going through these CBSE Class 11 Chemistry Notes Chapter 11 The p-Block Elements, students can recall all the concepts quickly.

The p-Block Elements Notes Class 11 Chemistry Chapter 11

→ General trends in the chemistry of p-block elements.

→ Group-13 Elements: The Boron family-Electronic configuration, atomic radii, ionization, enthalpy, electro-negativity, physical & chemical properties.

→ Important trends & anomalous properties of boron

→ Important compounds of boron: Borax, orthoboric acid, diborane, uses of boron & aluminium & their compounds.

→ Group-14 Elements: The carbon family. Electronic configuration, covalent radius, ionization enthalpy, electronegativity, physical & chemical properties.

→ Important trends & anomalous behaviour of carbon.

→ Allotropes of carbon: Diamond, graphite & fullerenes & uses of carbon.

→ Important compounds of carbon & silicon: Carbon monoxide, carbon dioxide, silicon dioxide, silicones, silicates & zeolites.

→ P-BIock Elements: p-block of elements of the periodic table is unique in terms of having all types of elements-metals, non¬metals & metalloids. Group numbers ranging from 13-18.

→ Valence shell electronic configuration ns².np^1-6(Except for He).

→ pπ-pπ bonds and dπ – pπ or dπ-dπ bonds: The combined effect of size & availability of d-orbitals considerably influences the ability of these elements to form π-bonds. While the lighter elements form pπ-pπ bonds. The heavier ones form dπ-dπ bonds.

→ Electron deficiency in boron compounds: The availability of 3-valence electrons for covalent bond formation using four orbitals (2S, 2P_x, 2P_y & 2P_z.) leads to the so-called electron deficiency in boron compounds.

→ Boranes: Boron forms covalent molecular compounds with di-hydrogen as boranes. The simplest is diborane (B₂H₆).

→ Inert pair effect: Aluminium exhibits + 3 oxidation state. With heavier elements, the +1 oxidation state gets progressively stabilised on going down the group. This is a consequence of the so-called inert pair effect.

→ Catenation: The ability to form chains or rings not only with C – C single bonds but also with multiple bonds
(C = C or C ≡ C).

→ Allotropes of carbon: Three important allotropes of carbon are diamond, graphite & fullerenes.

→ Carbon monoxide: Carbon monoxide having lone pair of electrons on C forms metal carbonyls. It is deadly poisonous due to the higher stability of its haemoglobin complex as compared to that of the oxy-haemoglobin complex.

→ Carbon dioxide: Increased content of CO₂ in the atmosphere due to combustion of fossil fuels & decomposition of limestone is feared to cause an increase in the greenhouse effect. This in turn raises the temperature of the atmosphere & causes serious complications.

→ Compounds of silicon: Silica, silicates & silicones are important compounds & find applications in industry & technology.

Chapter in Brief:
In the case of elements of p-block, the last electron enters a p-orbital. As p-subshell can hold a maximum of 6 electrons in p_x, p_y and p_z atomic orbitals, p-block has 6 groups namely 13, 14, 15, 16, 17 and 18th groups. The valence shell electronic configuration is ns² np^1-6. In the case of the boron family (group 13), carbon family (group 14) and nitrogen family (group 15), the group oxidation states (the most stable oxidation states) are +3, +4 and +5 respectively for the lighter element an in the respective groups.

However, the oxidation state two until less than the group oxidation state becomes increasingly more stable for the heavier elements in each group. The occurrence of oxidation state two units less than the group’s oxidation state is due to the Inert Pair Effect.

General Electronic Configuration And Oxidation States Of P-Block Elements

Non-metals and metalloids exist only in the p-block. The non-metallic character of elements decreases down a particular group. In fact, the heaviest element in each group of the p-block is the most metallic in nature.

In general non-metals have higher ionisation enthalpies and higher electronegativities than metals. Hence in contrast to metals which readily form cations, non-metals readily form anions. The compounds formed by highly reactive non-metals like halogens with highly reactive metals like alkali metals are generally Ionic due to the large difference in their electronegativities.

On the other hand, compounds formed by non-metals themselves are largely covalent because of the small differences in their electronegativities. The change of non-metallic to metallic character can be best illustrated by the nature of oxides formed by them. The non-metallic oxides like CO₂ and SiO₂ are acidic or neutral whereas metallic oxides like CaO Na₂O are basic.

The first member of the groups of p-block differs from the remaining members of their corresponding group in two major respects. First is the size and all other properties which depend on size. Thus, the lightest p-block elements show the same kind of differences as the lightest s-block elements, lithium and beryllium. The second important difference, which applies only to the p-block elements, arises from the effect of d-orbitals in the valence shell of heavier- elements (starting from the third period onwards) and their lack in second-period elements.

The second-period elements starting from boron are restricted to a maximum covalence of four (using 2s and three 2p orbitals). In contrast, the third-period element of a p-group with the electronic configuration 3s²3pⁿ has the vacant 3d orbitals lying between the 3p and the 4s levels of energy. Using these d-orbitals the third-period elements can expand their covalence above four. For example, while boron form only [BF₄]^–, aluminium gives [AlF₆]^3- ion. The presence of these d-orbitals influences the chemistry of the heavier elements in a number of other ways.

The combined effect of size and availability of d orbitals considerably influences the ability of these elements to form their bonds. The first member of a group differs from the heavier members in their ability to form pπ-pπ multiple bonds to itself (e.g., C = C, C ≡ C, N ≡ N) and to other second-row elements (e.g., C = 0, C = N, C ≡ N, N = 0). This type of π-bonding is not particularly strong for the heavier p-block elements. The heavier elements do form, n bonds but this involves d orbitals (dπ-pπ or dπ—dπ).

As the d orbitals are of higher energy than the p-orbitals, they contribute less to the overall stability of molecules than does pπ -pπ bonding of the second-row elements. However, the coordination number in species of heavier elements may be higher than for the first element in the same oxidation state. For example, in the +5 oxidation state both N and P form oxoanions:

NO^3- (with π-bonding involving one nitrogen p-orbital ) and PO₄^3- (four-coordination involving s,p and d orbitals contributing to the π-bonding).

Group 13 elements: The boron family
Boron, Aluminium, Gallium, Indium and Thallium are the elements present in group 13. Boron (B) is a typical non-metal. Aluminium is a metal. Gallium, indium and thallium are almost exclusively metallic in character.

Atomic & Physical Properties of Group 13 Elements


^aMetallic radius, ^b6-coordination, ^cPauling scale,

For M³⁺ (aq) + 3e^– → M(s)
^eFor M⁺ (aq) + e^– → M(s).

1. Electronic Configuration: The outer electronic configuration of these elements is ns²np¹

2. Atomic Radii: Generally atomic radii increase in going down the group. However atomic radius of Ga is less than that of Al, due to the poor screening effect of the inner d-electrons for the valence electrons from the increased nuclear charge in gallium.

3. Ionisation Enthalpy: IE of Al is less than that of B due to the increased size of Al.

4. Electronegativity: Electronegativity first decreases from B to Al and then increases marginally.

5. Physical Properties: Boron is non-metallic, extremely hard and black coloured solid. It exists in many allotropic forms. It has unusually high M.Pt. The rest of the members are soft metals with low M.Pt. and high electrical conductivity Gallium with M.Pt. of 303 K is a liquid during summer. The density of elements increases down the group.

6. Chemical Properties: Due to its small size the sum of its first three enthalpies is very high. Therefore B does not form +3 cations and forms only covalent bonds. Al due to its low I.E. forms Al³⁺ ions. In the heavier metals due to the inert pair effect, they exhibit an oxidation state of +1.

BF₃ is an electron-deficient compound and acts as a Lewis acid by accepting a pair of electrons.

AlCl₃ achieves stability by forming a dimer.

Trivalent covalent state compounds are hydrolysed by water to form tetrahedral [M(OH)₄]^– species, the hybridisation state of M is sp³. AlCl₃ in acidified aqueous state forms octahedral [Al(H₂O)₆]³⁺ ion. Al is in d²sp³ hybridisation.

1. Reactivity towards air: Boron is unreactive in crystalline form. A1 forms a very thin oxide layer on the surface which protects the metal from further attack. On heating B₂O₃ and Al₂O₃ are formed. With N₂, they form nitrides at a higher temperature.

2. Reactivity towards acids and bases: B does not react. Al dissolves in dilute HCl and liberates H₂ gas
2Al(s) + 6HCl(aq) → 2Al³⁺ (aq) + 6Cl^–(aq) + 3H₂(g)
Cone. HNO₃ renders A1 passive by forming a protective oxide layer on the surface.

Al reacts with aq. alkalies and liberates H2 gas.

3. Reactivity towards halogens.
2E(s) + 3X₂(g) → 2EX₃(S) (X = F, Cl, Br, I)
E = B, Al, Ga, In.

Important Trends and Anomalous Properties of Boron:
1. The trihalides of all these elements are covalent in nature and hydrolysed by water
EX₃ + 3H₂O → E(OH)₃ + 3HX

2. Monomeric trihalides, being electron deficient are strong LEWIS ACIDS.

3. Maximum covalency shown by boron is 4 because it cannot expand its octet beyond 4 due to the absence of d-orbitals. Due to the availability of d-orbitals with other metals, the maximum covalent can be expected beyond 4.

AlCl₃ is dimerised to AlCl₆

Some Important Compounds of Boron:
1. Borax: It is a white crystalline solid of formula Na₂B₄O₇.10H₂O, more appropriately Na₂[B₄O₅(OH)₄].8H₂O. It dissolves in water to give an alkaline solution.

2. Orthoboric Acid: It is a white crystalline solid with soapy touch. Its formula is H₃BO₃. It is sparingly soluble in water but highly soluble in hot water.

Preparation:

1. Na₂B₄O₇ (Borax) + 2HCl + 5H₂O → 2NaCl + 4B(OH)₃ (Boric acid)
2. It is formed by hydrolysis with water of BCl3:
   BCl₃ + H₂O(aq) → H₃BO₃ + 3HCl.

Structure: It has a layer structure in which planar BO₃ units are joined by hydrogen bonds as shown in the figure below.

[Structure of boric apid H₃BO₃ dotted line represent hydrogen bonds.]

Properties of Boric Acid (H₃BO₃)

1. It is a weak monobasic acid.
2. It is not a protonic acid but acts as Lewis-acid by accepting electrons from a hydroxyl ion
   B(OH)₃ + 2HOH → [B(OH)₄]- + H₃O⁺
3. On heating above 370K, metaboric acid (HBO₂) is formed which on further heating yields boric oxide (B₂O₃).
   
   Diborane B₂H₆: It is the simplest of boron hydrides.

Preparation:


(iii) Industrially it is prepared by the reaction of BF₃ on sodium hydride.

Properties of Diborane:
1. If is a colourless, highly toxic gas with a B.Pt. of 180 K.

2. It catches fire spontaneously upon exposure to air. Enormous energy is released during the reaction.
B₂H₆ + 3O₂ → B₂O₃ + 3H₂O; Δ_CH° = -1976 kJ mol”1

3. Most of the higher boranes are highly flammable.

4. It is hydrolysed by water giving boric acid
B₂H₆(g) + 6H₂O(l) → 2B(OH)₃(aq) + 6H₂O

5. Diborane undergoes cleavage reactions with Lewis bases to give borane adduct
B₂H₆ + 2NMe₃ → 2BH₃ . NMe₃
B₂H₆ + 2CO → 2BH₃ . CO
B₂H₆ + 2NH₃ → B₂H₆.2NH₃
which is formulated as [BH₂(NH₃)₂]⁺ [NH₄]^– further heating gives [BH₂(NH₃)₂]⁺ [BH₄]^– , further heating gives Borazine or Borazole or Inorganic Benzene B₃N₃H₆


The structure of diborane is shown in Fig.(a) below. The four-terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms. The four-terminal B—H bonds are regular two centre-two-electron bonds while the two bridge (B—H – B) bonds are different and can be described in terms of three centre-two electron bonds shown in Fig.(b)

(a) The strucwre of diborane, B₂H₆

Boron also forms a series of Hydridoborates; the most important one is the tetrahedral [BH₄]^– ion. Tetrahydridoborates of several metals is known. Lithium and sodium Tetrahydridoborates is also known as Borohydrides are prepared by the reaction of metal hydrides with B₂H₆ in diethyl ether.

(b) Bonding in diborane. Each B atom uses sp³ hybrids for bonding.

Out of the four sp³ Iribrids on each B atom, one is without an electron shown with broken lines. The terminal B-H bonds are normal 2 centre-2 electron bonds but lie two bridge bonds are 3 centre-2 electron bonds. The 3 centres 2 electron bridge bonds are also referred to as banana bonds.

2MH + B₂H₆ → 2M⁺[BH₄] ; M = Li or Na.
Both LiBH₄ and NaBH₄ are used as reducing agents in organic synthesis. They are starting materials for preparing other borohydrides.

Uses Of Boron & Aluminium And Their Compounds:
Boron is an extremely hard refractory solid of high melting point, low density and very low electrical conductivity find many applications. Boron fibres are used in making bullet-proof vest and light composite material for aircraft. The boron-10 (10B) isotope has a high ability to absorb neutrons and, therefore, metal borides are used in the nuclear industry as protective shields and control rods.

The main industrial application of borax and boric acid is in the manufacture of heat resistant glasses (e.g., Pyrex), glass-wool and fibreglass. Borax is also used as a flux for soldering metals, for heat, scratch and stain resistant glazed coating to earthenwares and as a constituent of medicinal soaps. An aqueous solution of orthoboric acid is generally used as a mild antiseptic.

Aluminium is a bright silvery-white metal, with high tensile strength. It has a high electrical and thermal conductivity. On a weight- to-weight basis, the electrical conductivity of aluminium is twice that of copper. Aluminium is used extensively in industry and everyday life.

It forms alloys with Cu, Mn, Mg, Si and Zn. Aluminium and its alloys can be given shapes of pipe, tubes, rods, wires, plates or foils and, therefore, find uses in packing, utensil making, construction, aeroplane and transportation industry. The use of aluminium and its compounds for domestic purposes is now reduced considerably because of its toxic nature.

Group 14 Elements: The Carbon Family
Carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb) are the members of group 14.
1. The valence shell electronic configuration of these elements is ns²np¹.

2. Covalent Radius: There is a considerable increase in covalent radius from C to Si, thereafter from Si to Pb, a small increase in radius is observed. This is due to the presence of completely filled d and f-orbitals in heavier members.

Atomic And Physical Properties Of Group 14 Elements:

^afor MIV oxidation state; ^b6-coordination, ^cPauling scale, ^d293 K; ^efor diamond; for graphite, density is 2.22; ^fβ-form (stable at room temperature)

3. Ionization Enthalpy: The first TE of group 14 members is higher than the corresponding members of group 13. It generally decreases from top to bottom. There is a small increase in the case of lead and it is due to the poor shielding effect of intervening d and f orbitals and the increase in the size of the atom.

4. Electronegativity: Due to the small size, the elements of this group are slightly more electronegative than group 13 elements. The electronegativity values for elements from Si to Pb are almost the same.

5. Physical Properties: All group 14 elements are solids, C and Si are non-metals, germanium (Ge) is a metalloid, whereas tin and lead are soft metals with low melting points. Melting points and boiling points of group 14 elements are much higher than those of the corresponding elements of the. group 13 elements.

6. Chemical Properties:
Oxidation states and trends in chemical reactivity: The common oxidation states shown by these elements are +4 and +2. Since the sum of four ionisation enthalpies is very high, compounds in the +4 oxidation state are generally covalent. The heavier members Ge, Sn and Pb, tendency to show an oxidation state of +2 increases due to the inert pair effect, i.e., the two electrons in ns² orbital prefer to remain paired if we go down the group and do not participate in bond formation.

C & Si mostly show an oxidation state of + 4.

Ge shows a + 4 states in stable compounds and only a few compounds in a + 2 oxidation state.
Sn forms compounds in both oxidation state + 4 and + 2 (Sn in + 2 states is a reducing agent)

Lead compounds in the + 2 state are stable and in the + 4 states are strong oxidising agents.
Being electron-precise molecules, they are neither electron- acceptors nor electron-donors.

Although C cannot expand its octet beyond 4 due to the non-availability of d-orbitals, other elements of the group can do so, because of the presence of d-orbitals in them. CCl₄ can’t undergo hydrolysis, whereas SiCl₄ can do so due to the same reason.

For examples, the species like SiF₅^–, SiF₆^2-, GeCl₆^2- and [Sn(OH)₆]^2- exist where the hybridisation of the central atom is sp²d³.
1. Reactivity towards oxygen: All members on heating in oxygen form oxides-MO and MO₂. Oxides in a higher oxidation state are more acidic than in a lower oxidation state. CO is neutral, CO is acidic. The dioxides-CO₂, SiO₂, GeO₂ are acidic, SnO₂ and PbO₂ are amphoteric. GeO is distinctly acidic, SnO and PbO are amphoteric.

2. Reactivity towards water:
C, Si, and Ge are not affected by water

Pb is not affected by water.

3. Reactivity towards halogens
M + X₂ → MX₂ M: Si, Ge, Sn, Pb
M + 2X₂ → MX₄ X: F, Cl, Br, I

Most MX₄ are covalent M shows sp3 hybridisation and MX₄ are tetrahedral in shape. SnF₄ & PbF₄ are ionic in nature. Pbl4 does not exist. Stability of MX₂ increases down the group.

GeX₄ is more stable than GeX₂, whereas PbX₂ is more stable than PbX₄. Sid₄ undergoes hydrolysis as shown below, but CCl₄ cannot undergo hydrolysis because carbon cannot expand its covalence beyond four due to the absence of d-orbitals

Important Trends and Anomalous Behaviour of C:
Carbon (C) the first member of group 14 differs from its congeners due to

1. Small size
2. Higher ionisation enthalpy and higher electronegativity.
3. Non-availability of d-orbitals.

In carbon, only s and p orbitals are available for bonding and, therefore, it can accommodate only four pairs of electrons around it. This would limit the maximum covalence to four whereas other members can expand their covalence due to the presence of d orbital.

Carbon also has a unique ability to form pπ-pπ multiple bonds with itself and with other atoms of small size and high electronegativity. Few examples of multiple bonding are: C = C, C ≡ C, C=0, C = S, and C = N. Heavier elements do not form pπ-pπ bonds because their atomic orbitals are too large and diffuse to have effective overlapping.

Carbon atoms have the tendency to link with one another through covalent bonds to form chain and rings. This property is called catenation. This is because C—C bonds are very strong. Down the group the size increases and electronegativity decreases, and, thereby, the tendency to show catenation decreases. This can be clearly seen from bond enthalpies values. The order of catenation is C >> Si > Ge = Sn. Lead does not show catenation.

Due to the property of catenation and pπ-pπ bonds formation, carbon is able to show allotropic forms.

Allotropes of Carbon:
Carbon exists in crystalline and amorphous forms. Diamond and graphite are two well-known crystalline forms of carbon. In 1985, the third form of C known as Fullerenes was discovered:

The structure of diamond

Carbon in diamond is sp³ hybridised. Diamond has a crystal lattice. The C—C bond length is 154 pm. The structure is a rigid three-dimensional network of carbon atoms. In this structure shown on the side, directional covalent bonds are present throughout the lattice.

It is very difficult to break extended covalent bonding and therefore diamond is the hardest substance on the earth. It is used as an abrasive for sharpening hand tools, in making dies and in the manufacture of tungsten filaments for electric light bulbs.

Graphite:
Graphite has a layered structure. Layers are held by van der Waals forces and the distance between the two layers is 340 pm. Each layer is composed of planar hexagonal rings of C atoms. C—C bond length within a layer is 142 pm. Here C undergoes sp² hybridisation and makes three bonds with 3 neighbouring C atoms. The fourth electron forms a bond. The electrons are delocalised over the whole sheet.

These electrons in graphite are mobile and therefore, graphite conducts electricity- Graphite is very soft and is used as a dry lubricant in machines running at high temperature, where oil cannot be used as a lubricant.

Structure of graphite

Fullerenes:
Fullerenes are made by heating graphite in an electric arc in the presence of an inert gas such as helium or argon. The sooty material formed by the condensation of vapourised C₆₀ small molecules consists up mainly of a smaller quantity of C₇₀ and traces of fullerenes consisting of an even number of carbon atoms up to 350 or above. Fullerenes are the only pure forms of carbon because they have smooth structure without having “dangling” bonds.

Fullerenes are cage-like molecules. the molecule has a shape like a soccer ball and is called Buckminster Fullerene. It contains twenty six-membered rings and twelve five-membered rings. A six-membered ring is fused with six or five-membered rings but a five-membered ring can only fuse with six-membered rings.

All the carbons atoms are equal and they undergo sp³ hybridisation. Each carbon atom forms three sigma bonds with the other three carbon atoms. The remaining electron at each carbon atom is delocalised in molecular orbitals which give an aromatic character to the molecule.

This ball-shaped molecule has 60 vertices and each one is occupied by one C atom and it contains both single and double bonds with C-C distances of 143.5 pm and 138.3 pm respectively. Spherical fullerenes are also called Bucky Balls

The structure of C 60, Buckminster fullerene. Note that molecule has the shape of a soccer ball (football)

Graphite is a thermodynamically most stable allotrope of carbon and therefore Δ_fH° of graphite is taken as zero.

Uses of Carbon:

1. Graphite fibres embedded in plastic material form high strength, lightweight composites which find wide applications.
2. Being a good conductor, graphite is used as electrodes in batteries and in industrial electrolysis.
3. Crucibles made of graphite are inert to dilute acids and alkalies.
4. Graphite is used as a moderator in nuclear reactors to slow down the speed of fast-moving neutrons.
5. Being highly porous activated charcoal is used in absorbing poisonous gases. It is also used in water filters to remove organic contaminators and in the air conditioning system to control odour.
6. Carbon black is used as a black pigment in black ink and as filler in automobile tyres.
7. Coke is used as a fuel and largely as a reducing agent in metallurgy.
8. Diamond is a precious stone and used in jewellery. It is measured in carat (1 carat = 200 mg)

Some Important Compounds of Carbon and Silicon:

• Oxides of Carbon: Two important oxides of C are carbon monoxide CO and carbon dioxide CO₂.
• Carbon Monoxide (CO): Direct oxidation of carbon in a limited supply of air or oxygen yields CO.

1. Lab. method: On a small scale CO is prepared by dehydration of formic acid with cone. H₂SO₄ at 373K

2. Commercial-scale: It is prepared commercially by the passage of steam over hot coke. The mixture of CO and H₂ produced is called water-gas or synthesis gas

When air is used instead of steam a mixture of CO and N₂ produced which is called producer gas.

Properties:

1. It is a colourless odourless gas.
2. It is almost insoluble in water;
3. It- is a powerful reducing agent and reduces all metal oxides other than those of alkali and alkaline earth metals, aluminium and a few transition elements.
   
4.  In C ≡ O: there is one sigma and two n bonds between C and oxygen. Because of the presence of a lone pair of electrons on C, the CO molecule acts as a donor and reacts with certain metals when heated to form metal carbonyls.
   
5. Due to its highly poisonous nature, CO forms a complex with haemoglobin which is about 300 times more stable than the oxygen complex. This prevents haemoglobin in the red blood corpuscles from carrying oxygen around the body and ultimately results in death.

Carbon Dioxide:
Methods of Preparation.

1. Complete combustion of C and C containing fuels.
   
2. Lab. method
   CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)
3. Commercially, it is prepared by heating lime stone,
   

Properties:

1. It is colourless and odourless gas.
2. It has low solubility in water. With water, it forms carbonic acid H₂CO₃ which is a weak dibasic acid.
   H₂CO₃^– + H₂O ⇌ HCO₃^– + H₃O⁺
   HCO₃^– + H₂O ⇌ CO₃^2- + H₃O⁺
3. 2NaOH + CO₂ → Na₂CO₃ + H₂O
4. Photosynthesis
   
5. Excess of CO₂ in the atmosphere leads to the greenhouse effect which will raise the temperature of the atmosphere.
6. CO₂ in the solid state is called Dry ice which is used as a refrigerant for ice cream and frozen food.

Structure of CO₂
C in CO₂ undergoes sp hybridisation. Two sp hybridised orbitals of carbon atom overlap with two p orbitals of oxygen atoms to make two sigma bonds while the other two electrons of the carbon atom are involved in pπ-pπ bonding with an oxygen atom. This results in its linear shape [with both C-O bonds of equal length (115 pm)] with no dipole moment. The resonance structures are shown below:

Resonance structures of carbon dioxide

Silicon Dioxide SiO₂
Silicon dioxide or silica along with silicates constitute 95 % of the earth’s crust. SiO₂ is a covalent three-dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms. Each oxygen atom in turn covalently bonded to another silicon atoms as shown.

Three-dimensional structure of SiO

Properties:

1. Silica in its normal state is almost non-reactive.
2. It is attacked by HF and NaOH.
   SiO₂ + 4HF → SiF₄ + 2H₂O
   SiO₂ + 2NaOH → Na₂SiO₃ (Sodium silicate) + H₂O

Uses: Silica gel is used as a drying agent, as a catalyst and in chromatography.

Silicones: They are a group of organosilicon polymers that have -R₂SiO- as a repeating unit. They are prepared as follows:

industries for cracking of hydrocarbons and isomerisation, e.g., ZSM-5 (A type of zeolite) used to convert alcohols directly into gasoline. Hydrated zeolites are used as ion exchangers in softening hard water.