Prasanna

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.