CBSE Notes

By going through these CBSE Class 12 Chemistry Notes Chapter 14 Biomolecules, students can recall all the concepts quickly.

Biomolecules Notes Class 12 Chemistry Chapter 14

Carbohydrates: Most common examples of carbohydrates are glucose, fructose, cane sugar, starch etc. Most of them have a general formula C_x (H₂O)_y. Earlier they were considered hydrates of carbon. For example, glucose C₆H₁₂O₆ fits into this general formula C₆(H₂O)₆ But even acetic acid (CH₃COOH) fits into this general formula C₂(H₂O) and it is not a carbohydrate. Similarly, rhamnose, C₆H₁₂O₅ is a carbohydrate but does not fit into this definition.

Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.

They are classified as:

1. Sugars: They are sweet in taste and water-soluble, e.g. glucose, fructose, sucrose.
2. Non-sugars: They are tasteless and water-insoluble, e.g., starch, cellulose. Carbohydrates are systemically classified as:

1. Monosaccharides: A carbohydrate that cannot be hydrolysed further to give simpler units of polyhydroxy aldehydes or ketones is called monosaccharides. Glucose (C6H1206) is an aldohexose and fructose (C6H1206) is a ketohexose.

2. Oligosaccharides: Carbohydrates that yield two to ten monosaccharides on hydrolysis are called oligosaccharides.
(a) Disaccharides: They hydrolyse to give two units of monosaccharides. They include sucrose, maltose, lactose.

(b) Trisaccharides: They yield three units of monosaccharides on hydrolysis, e.g. C₁₈H₃₂O₁₆ (raffinose).
(c) Tetrasaccharides: Yields four units of monosaccharides on hydrolysis, e.g. stachyose C₂₄H₄₂O₂₁

2. Polysaccharides: They yield a large number of monosaccharide units on hydrolysis: Common examples are starch, cellulose. They are not sweet in taste.

Reducing sugars are those which reduce Fehling’s solution and Tollen’s reagent. All monosaccharides whether aldoses and ketoses are reducing sugars.

Sugars that do not reduce Fehling solution or Tollen’s reagent are termed as non-reducing e.g., sucrose.

→ Monosaccharides: They contain three to seven carbon atoms. If they contain an aldehyde group (- CHO), they are termed aldoses. If they contain a keto group (C = O), they are termed ketoses.

Different Types of Monosaccharides:

1. Glucose:
Preparation:
(a) From Sucrose (Cane Sugar)

(b) From Starch:

→ Structure of Glucose: It is an aldohexose and is also known as dextrose. Its structure (open chain) is

Evidence in favour of the above structure:

1. Its molecular formula was determined to be C₆H₁₂O₆.
2. On heating (prolonged) with HI, it formed an n-hexane suggesting that all the 6 carbon atoms are in a straight chain.
   
3. It reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide (HCN) to give cyanohydrin showing the presence of a carbonyl group in it,
   
4. Glucose is oxidised to gluconic acid by mild Oxidizing agent Br. water, confirming that a carbonyl group is an aldehyde group.
   
5. Acetylation with acetic anhydride gives glucose pentaacetate which confirms the presence of five – OH groups attached to 5 different C atoms.
   
6. on oxidation with nitric acid, glucose well as gluconic acid both yield a dicarboxylic acid, saccharic acid indicating the presence of -CH₂OH group in it in addition to an aldehyde.
   

The exact spatial arrangement of different – OH groups was given by Fischer. Its exact configuration is correctly represented by I. Gluconic acid is II and Saccharic acid is III.

Glucose is correctly named as D (+) glucose. ‘D’ represents the configuration whereas (+) represents the dextro-rotatory nature of it. The meaning of D- and L- notations is given as follows:

[Note: It may be remembered that ‘D’ and ‘L’ notations have nothing to do with the optical activity of the compound.]

The letters ‘D’ or ‘L’ before the name of any compound indicate the relative configuration of a particular stereoisomer. This refers to their relationship with a particular isomer of glyceraldehyde. Glyceraldehyde contains one asymmetric carbon atom and exists in two enantiomeric forms as shown below.

All those compounds which can be chemically correlated to (+) isomer of glyceraldehyde are said to have D-configuration whereas those which can be correlated to (-) isomer of glyceraldehyde are said to have L—configuration.

For assigning the configuration of monosaccharides, it is the lowest asymmetric carbon atom (as shown below) which is compared. As in (+) glucose, —OH on the lowest asymmetric carbon is on the right side which is comparable to (+) glyceraldehyde, so it is assigned D-configuration. For this comparison, the structure is written in a way that most oxidised carbon is at the top.

Cyclic Structure of Glucose

The structure (I) of glucose explained most of its properties but the following reactions and facts could not be explained by this structure.

1. Despite having the aldehyde group, glucose does not give 2,4- DNP test, Schiff’s test and it does not form the hydrogen sulphite addition product with NaHSO₃.
2. The pentaacetate of glucose does not react with hydroxylamine indicating the absence of the free -CHO group.
3. Glucose is found to exist in two different crystalline forms which are named a and b. The a-form of glucose (m.p. 419 K) is obtained by crystallization from a concentrated solution of glucose at 303 K while the (i-form (m.p. 423 K) is obtained by crystallisation from hot and saturated aqueous solution at 371 K,

This behaviour could not be explained by the open-chain structure (I) for glucose. It was proposed that one of the -OH groups may add to the -CHO group and form a cyclic hemiacetal structure. It was found that glucose forms a six-membered ring in which -OH at C-5 is involved in a ring formation. This explains the absence of -CHO group and also the existence of glucose in two forms as shown below. These two cyclic forms exist in equilibrium with an open-chain structure.

The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at Cl, called anomeric carbon (the aldehyde carbon before cyclization). Such isomers, i.e., a-form and b-form, are called anomers.

The six-membered cyclic structure of glucose is called the pyranose structure (α- or β-), in analogy with pyran. Pyran is a cyclic organic compound with one oxygen atom and five carbon atoms in the ring. The cyclic structure of glucose is more correctly represented by Haworth structure as given below:

II. Fructose
Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose. It has a ketonic group at C – 2. It belongs to D-series and is a laevorotatory compound. Therefore, it is written as D – (-) fructose. Its open-chain structures are given below:

→ It differs from glucose only at C – 1 and C – 2. Its furanose form (cyclic) is:

→ The cyclic structures of two anomers of fructose as represented by Haworth are given below:

Disaccharides:
1. Sucrose: Sucrose on hydrolysis gives an equimolar mixture of D – (+) – glucose and D – (-) fructose.

Sucrose is a non-reducing sugar. Therefore, it has a glucoside linkage between C₁ of α-glucose and C₂ of β-fructose.

or

Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (- 92.4°) is more than the dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus hydrolysis of sucrose brings about a change in the sign of rotation, from Dextro (+) to leave (-) and the product is named as invert sugar.

II. Maltose: Another disaccharide, maltose is composed of two α-D-glucose units in which C₄ of one glucose (I) is linked to C₄ of another glucose unit (II). Hie free aldehyde group can be produced at C₁ of second glucose in solution and it shows reducing properties, so it is a reducing sugar.

II. Lactose: It is more commonly known as milk sugar since this disaccharide is found in milk. It is composed of (β-D-galactose and β-D- glucose. The linkage is between C₄ of galactose and C₄ of glucose. Hence it is also a reducing sugar.

Polysaccharides: Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages.
I. Starch: Starch is the main storage polysaccharide of plants. It is a polymer of a-glucose and consists of two components 15-20% of water-soluble Amylose and Amylopectin which is water-insoluble and constitutes about 80-85% of starch. Their structures have been given below:

II. Cellulose: Cellulose occurs exclusively in plants. It is a predominant constituent of the cell walls of plant cells. Cellulose is a straight-chain polysaccharide composed of only β-D-glucose units which are joined by the glycosidic linkage between C₁ of one glucose unit and C₄ of the next glucose unit.

III. Glycogen: The carbohydrates are stored in the animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is more highly branched.

→ Proteins: Proteins are the most abundant biomolecules of the living system. Chief sources of proteins are milk, cheese, pulses, peanuts, fish and meat etc. They are required for the growth and maintenance of the body. All proteins are polymers of a-amino acids.

→ Amino acids: Amino acids contain an amino (- NH₂) and carboxyl (- COOH) functional groups.

→ Classification of Amino acids: Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. An equal number of amino and carboxyl groups makes it neutral; more amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic.

The amino acids, which can be synthesized in the body, are known as non-essential amino acids. On the other hand, which cannot be synthesized in the boxy and must be obtained through diet, are known as essential amino acids (marked with an asterisk in Table below).

Amino acids are usually colourless, crystalline solids. These are water-soluble, high melting solids and behave like salts rather than simple amines or carboxylic acids. This behaviour is due to the presence of both an acidic (carboxyl group) and a basic (amino group) group in the same molecule. In an aqueous solution, the carboxyl group can lose a proton and the amino group can accept a proton, giving rise to a dipolar ion known as a zwitterion. This is neutral but contains both positive and negative charges.

In zwitterionic form, amino acids show amphoteric behaviour as they react both with acids and bases.

Except for glycine, all other naturally occurring a-amino acids are optically active. These exist both in D and L forms. Most naturally occurring amino acids have L-configuration. L-Amino acids are represented by writing the – NH₂ group on the left hand.

Table: Natural Amino Acids,



→ Structures of Proteins: Proteins are the polymers of a-amino adds linked through peptide bond or peptide linkage.

If a third amino acid combines with a dipeptide, the product is called a tripeptide. When the number of such amino acids is more than 10, then the products are called polypeptides. A polypeptide with more than 100 units of amino acid residues, having a molecular mass higher than 10,000 u is called a protein.

Proteins can be classified into two types:
(a) Fibrous proteins: When the polypeptide chains run parallel and held together by hydrogen and disulphide bonds, then a fibre-like structure is formed. Such proteins are generally insoluble in water.

(b) Globular proteins: This structure results when the chains of polypeptides coil around to give a spherical shape. These are usually soluble in water.

Insulin and albumins are common examples.
1. Primary structure of Proteins: Proteins may have one or more polypeptide chains. Each polypeptide is a protein that has amino acids linked with each other in a specific sequence and it is this sequence of amino acids that are said to be the primary structure of that protein.

2. Secondary structure of Proteins: The secondary structure of a protein refers to the shape in which a long polypeptide chain can exist. They are found to exist in two different types of structures, viz., a-helix and P-pleated sheet structure.

3. The tertiary structure of protein represents overall folding of the polypeptide chains i.e., further folding of the secondary structure. It gives rise to two major molecular shapes viz. fibrous and globular. The main forces which stabilise the 2° and 3° structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction.

4. Quaternary Structure of Proteins: Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as a quaternary structure.

A diagrammatic representation of all these four structures is given in the figure below:

→ Denaturation of Proteins: When a protein in its native form is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. The protein loses its biological activity. This is called denaturing of proteins, 2° and 3° structures are destroyed, but 1° structure remains intact. The coagulation of egg white on boiling is a common example.

→ Enzymes: The enzymes are biological catalysts produced by living cells that catalyse biochemical reactions. The enzymes differ from other types of catalysts in being highly specific and selective.

→ Mechanism of Enzyme Action: Enzymes, like catalysts, are needed only in small quantities and reduce the magnitude of activation energy of the activated complex. For example, the activation energy for acid hydrolysis of sucrose is 6.22 kJ mol^-1 which is reduced to 2.15 kJ mol^-1 when hydrolysed by the enzyme sucrase.

→ Vitamins: Certain organic compounds are required in small amounts in our diet but their deficiency in the body causes specific diseases. These compounds are called vitamins. In small quantities in the diet perform specific biological functions for normal maintenance of optimum growth and health of the organism.

Classification of Vitamins:

1. Fat-soluble Vitamins: Vitamins like A, D, E and K are fat or oil-soluble, but insoluble in water. They are stored in the liver and adipose tissues.
2. Water-soluble Vitamins: B group Vitamins and Vitamin C are soluble in water. They (except vitamin B12) cannot be stored in a body.

→ Nucleic acids: The particles in the nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of biomolecules called nucleic acids. They are mainly of two types, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since nucleic acids are long-chain polymers of nucleotides, so they are also called polynucleotides.

→ Chemical composition of Nucleic acids: Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen

Table: Vitamins, their sources and their deficiency diseases:

containing heterocyclic compounds called bases. In DNA molecules, the sugar part is β-D-2-deoxyribose whereas, in the RNA molecule, it is β-D- ribose.

DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also contains four bases, the first three bases are A, G and C (as in DNA), but the fourth base is Uracil (U).

→ Structure of.Nucleic acids: A unit formed by the attachment of a base to the 1′ position of sugar is known as a nucleoside. In nucleosides, the sugar carbons are numbered as 1′, 2′, 3′ etc in order to distinguish these from the bases (Fig. (a) below). When nucleoside is linked to phosphoric acid at 5′-position of sugar moiety we get a nucleotide (Fig. (b) below)

(a) Structure of a nucleoside
(b) Structure of a nucleotide.

Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar. The formation of a typical dinucleotide is:

(Formation of a dinucleotide)

A simplified version of the nucleic acid chain is shown below:

RNA molecules are of three types and they perform different functions. They are named messenger RN A (m-RNA), ribosomal RNA (rRNA) transfer RNA (f-RNA).

DNA Fingerprinting is now used:

• in forensic laboratories for the identification of criminals.
• to determine the paternity of an individual.
• to identify the dead bodies in an accident by comparing the DNAs of parents or children.
• to identify racial groups to rewrite biological evolution.

Biological Functions of Nucleic Acids: DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. A DNA molecule is capable of self-duplication during cell division, and identical DNA strands are transferred to daughter cells.

Another important function of nucleic acids is the protein synthesis in the cell. Actually, the proteins are synthesised by various RNA molecules in the cell but the message is if the synthesis of a particular protein is present in DNA.

The first one is called Replication and the second one is called protein synthesis.

1. Replication: The process by which a single DNA molecule produces two identical copies of itself is called cell division or replication. Replication of DNA is an enzyme catalysed process.
2. Synthesis of Proteins: Another important function of DNA is the synthesis of proteins. In fact, DNA may be regarded as the instrument manual for the synthesis of all the proteins present in a cell.

The DNA directed synthesis of proteins occurs in the following two steps:

1. Transcription,
2. Translation

1. Transcription: It involves copying of DNA base sequence into an RNA molecule called the messenger RNA (m RNA).

2. Translation: The mRNA directs protein synthesis in the cytoplasm of the cell with the help of r RNA and t RNA. The process is called translation.

By going through these CBSE Class 12 Chemistry Notes Chapter 13 Amines, students can recall all the concepts quickly.

Amines Notes Class 12 Chemistry Chapter 13

Amines: Amines can be considered as derivatives of ammonia, obtained by replacement of H of NH₃ by alkyl /aryl group.

→ Structure of Amines: Like ammonia, the N atom of amines is trivalent and Carnes an unshared pair of electrons. N orbitals in amines are sp3 hybridized and the geometry of amines is pyramidal.

(Pyramidal shape of trimethylamine)

→ Classification of Amines: Amines are classified as primary (1° secondary (2°) and tertiary (3°) depending upon the no. of hydrogen atoms replaced by alkyl/aryl groups in ammonia molecule.

Amines are simple if R = R’ = R”
They are termed as mixed when R, R’, R” are different.

→ Nomenclature of Amines: In the IUPAC system ‘e’ of the alkane is replaced by ‘amine’ like Alkanamine. The simplest arylamine is aniline (C₆H₅—NH₂) or benzene amine

→ Preparation of Amines:
1. Reduction of Nitro Compounds

2. Ammonolysis of Alkyl Halides

The order of reactivity of halides is RI > RBr > RCI.

3. Reduction of Nitriles

4. Reduction of amides

5. Gabriel Phthalimide Synthesis: This method is used to prepare pure primary amines. Aromatic amines cannot be prepared by this method.

Hoffman Bromamide Degradation Reaction: Primary amines are obtained when an amide is treated with Br₂ in an aqueous solution of NaOH.

Physical Properties:

1. Lower aliphatic amines are gases with a fishy odour. Primary amines with three or more C atoms are liquids and still higher ones are solids.
2. Lower aliphatic amines are soluble in water because they can form H-bonds with water molecules. Higher amines are essentially insoluble in water.
3. The order of boiling points of isomeric amines is Primary > Secondary > Tertiary. It is due to the presence of intermolecular H-bonding which is more in primary amines (due to the presence of two H- atoms attached to N than only one in secondary amines), less in 2° amines and absence in tertiary amines.
   

Chemical Reactions:
1. Basic character of Amines: Amines being basic in character (Lewis bases) react with acids to form salts.

The basic character of amines can be better understood in terms of their P^K_b and k_b values as explained below:


The larger the value of K_b or lower the value of P^K_b, the stronger is the base. P^K_b value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to the + I-effect of alkyl groups. The availability of lone pair of electrons on N increases. On the other hand, aromatic amines are weaker bases than ammonia due to the electron-withdrawing nature of the aryl group. The P^K_b values of few amines are given below:

Table: P^K_b values of Amines in Aqueous Phase:

Besides the + I or – I effect, the interplay of other factors like solvation effect, steric hindrance etc. affect the basic strength of amines.

→ Structure-Basicity Relationship of Amines: The basicity of amines is related to their structure. The basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid.

The more stable the cation is relative to the amine, the more basic is the amine.
(a) Alkamines versus Ammonia
Let us consider the reaction of an alkanamine and ammonia with a proton to compare their basicity.

Due to the electron releasing nature of the alkyl group, it (R) pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton. Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +1 effect of the alkyl group. Hence, alkylamines are stronger bases than ammonia.

Thus the basic nature of aliphatic amines should increase with the increase in the number of alkyl groups. This trend is followed in the gaseous, phase. The order of basicity of amines in the gaseous phase follows the expected order: tertiary amine > secondary amine > primary amine > NH₃.

The trend is not regular in the aqueous state as evident by their P^K_b values given in Table. In the aqueous phase, the substituted ammonium cations get stabilised not only by the electron releasing effect of the alkyl group (+1) but also by hydrogen bonds and solvation with water molecules. The greater the size of the ion, the lesser will be the solvation and the less stabilised the ion. The order of stability of ions are as follows:

Decreasing order of extent of H-bonding in water and stabilization by solvation: Greater the stability of the substituted ammonium cation, the stronger base is the corresponding amine. Thus the order of basicity of aliphatic amines should be primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small like – CH₃ group, there is no steric hindrance to H-bonding.

In case the alkyl group is bigger than the CH₃ group, there will be some steric hindrance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from CH₃ to – C₂H₅ results in a change in the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hindrance of the alkyl group which decides the basic strength of alkylamines in the aqueous state.

The order of basic strength in the case of methyl-substituted amines and ethyl substituted amines in an aqueous solution is as follows:
(C₂H₅)₂ NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃
(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

(b) Arylamine Vs Ammonia: High value of P^K_b of aniline suggests that it is a much weaker base than aliphatic amines or even ammonia. It is due to the resonance shown by aniline.

Aniline is a resonance hybrid of 5 canonical structures out of which there is no lone pair of electrons available for sharing on N in II, III and IV structures. It results in the unshared electron pair on N being in conjugation with the benzene ring making it less available for protonation.

On the other hand, aclidinium ion obtained by accepting a proton can have only two resonating structures (Kekule) as shown below:

The greater the no, of resonating structures, the greater is the stability. Thus aniline is more resonance stabilized than anilinium ion. Hence the proton acceptability or the basic nature of aniline and other aromatic amines would be less than ammonia.

In the case of substituted aniline, electron-releasing groups like – OCH₃ – CH₃ increase the basic strength whereas electron-withdrawing groups like – NO₂, – SO₃H, – COOH, – X decrease it.
Thus (C₂H₅)₂ NH > C₂H₅NH₂ > NH₃ > C₆H₅NH₂

2. Alkylation: Amines undergo alkylation with alkyl halides.

3. Acylation: Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides and esters by a nucleophilic substitution reaction. This reaction is known as acylation.

It is the replacement of H of – NH₂ group
or

group by the acyl group.


The reaction of amines with benzoyl chloride (CH₅ COCl) is called Benzoylation.

4. Carbylamine reaction: It is a test of primary amines-both aliphatic and aromatic.

5. Reaction with Nitrous acid: All the three classes of amines react differently with nitrous acid, HNO₂ (prepared in situ from HCl + NaNO₂)


Secondary and tertiary amines react differently.

6. Reaction with aryl sulphonyl chloride: Benzensulphonyl chloride (C₆H₅SO₂Cl) called Hinsberg’s reagent reacts with 1° and.2° amines.

(c) Tertiary amines (R₃N) do not react with benzene sulphonyl chloride. This property of 1°, 2° and 3° amines is made use of in their distinction and separation Now in place of benzene sulphonyl chloride,

p-toluenesulphonyl chloride is used.

7. Electrophilic Substitution: NH₂ group fused in the ring is a powerful activating group and is ortho and para directing.
(a) Bromination:

Protection of the highly activating -NH₂ group can be done by acetylation with acetic anhydride in which case only monobromo substituted aniline is the product.

The lone pair of electrons on N of acetanilide interacts with oxygen atom due to resonance as shown below:

Hence the lone pair of electrons on nitrogen is less readily available for donation to the benzene ring by resonance. Thus the activating effect of – NHCOCH₃ is less than that of the – NH₂ group.

(b) Nitration: Direct nitration of aniline yields tarry oxidation products in addition to nitro derivatives.

However, by protecting the amino group by acetylation reaction with acetic anhydride the nitration can be controlled and the para nitro derivative obtained as the major product.

(c) Sulphonation:

Aniline does not undergo Friedel Crafts reaction (alkylation and acetylation) due to the salt formation with AlCl₃– the Lewis acid-which is used as a catalyst. The N of -NH₂ acquires a positive charge and hence acts as a strong deactivating group for further reaction.

Diazonium Salts are of the type Ar N₂⁺ X^– where Ar stands for an aryl group and X^– can be Cl^–, Br, HSO₄^–, BF₄^– etc. N₂⁺ group is called the diazonium group.

C₆H₅ N₂⁺Cl^– is called benzene diazonium chloride and C₆H₅ N₂⁺ HSO₄^– is known as benzene diazonium hydrogen sulphate. The stability of the arene diazonium ion is explained on the basis of resonance.

Method of Preparation:

Physical Properties:
1. It is a colourless crystalline solid.
2. It is readily soluble in cold water and stable in it but reacts with water when warmed.
3. It decomposes easily in the dry state. Therefore it is used immediately after its preparation and is not stored.

Chemical Reactions A: Reactions involving displacement of Nitrogen -N₂⁺ group is a very good leaving group. Therefore, it is substituted by other groups such as Cl^–, Br^–, I^–, CN^– and OH^– which displace nitrogen from the aromatic ring.
1. Replacement by halide or cyanide ion: Sandmeyer Reaction

Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating diazonium salt solution with corresponding halogen acid in the presence of copper powder. This is referred to as the Gatterman reaction.

The yield in the Sandmeyer reaction is found to be better than the Gattermann reaction.

2. Replacement by Iodide Ion: Iodine is not easily introduced into the benzene ring directly, but, when the diazonium salt solution is treated with potassium iodide, iodobenzene is formed.
Ar N₂⁺ Cl^– + KI → Arl + KCl + N₂

3. Replacement by Fluoride Ion: When arene diazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride.

4. Replacement by H: Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanol, respectively.
Ar N₂⁺ Cl^– + H₃PO₂ + H₂O → ArH + N₂ + H₃PO₃ + HCl
Ar N₂⁺ Cl^– + CH₃CH₂OH → ArH + N₂ + CH₃CHO + HCl

5. Replacement by hydroxyl group: If the temperature of the diazonium salt solution is allowed to rise up to 283 K, the salt gets hydrolysed to phenol.
Ar N₂⁺ Cl^– + H₂O → ArOH + N₂ + HCl

6. Replacement by – NO₂ group: When diazonium fluoroborate is heated with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by the NO₂ group.

(B) Reactions involving retention of diazo group: Coupling Reactions
(a) Reaction with Phenol in the presence of a weakly alkaline medium results in the formation of p-hydroxy azobenzene which is an orange dye.

(b) Reaction with aniline: It reacts with aniline in a weakly acidic medium to form p-amino azobenzene (yellow dye)

→ Importance of Diazonium salts in the synthesis of Aromatic Compounds: These salts are very good intermediates used to introduce – F, – Cl, – Br,-I, – CN, – OH, – NOz groups into the benzene ring. Ar-F or Ar-I cannot be prepared by direct halogenation. The cyanobenzene which cannot be prepared by the S_N reaction of chlorobenzene can be easily obtained from diazonium salts. These compounds are useful for preparing several azo dyes by coupling reactions.

By going through these CBSE Class 12 Chemistry Notes Chapter 12 Aldehydes, Ketones and Carboxylic Acids, students can recall all the concepts quickly.

Aldehydes, Ketones and Carboxylic Acids Notes Class 12 Chemistry Chapter 12

→ Aldehydes and Ketones both contain carbonyl C = O group and hydrogen while in Ketones, it is bonded to two carbon atoms.

→ Carbon in both aldehydes and Ketones is sp² hybridized:
If the Carbonyl group Carboxylic acid. is attached to -OH group, it is called Carboxylic Acid


Common names:

IUPAC Names: The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending – e with – al and – one respectively.


Table: Common and IUPAC Names of Some Aldehydes and Ketones:


→ Structures of the Carbonyl group): The carbonyl carbon atom is sp² hybridized and forms three sigmas (σ) bonds.

C = O is polarised due to the higher electronegativity of oxygen relative to carbon. Hence the carbonyl carbon is an electrophilic (Lewis acid) and carbonyl oxygen, a nucleophile (Lewis base) centre.

The carbonyl group is highly polar.

Preparation of Aldehydes and Ketones:
1. Aldehydes are prepared by the partial oxidation of primary alcohols while ketones are obtained from secondary alcoholic.

2. Catalytical dehydrogenation of primary alcohols with red hot Cu gauze at 573 K gives aldehydes and secondary alcohols give ketones.

3. From ozonolysis of alkenes:


4. From Alkynes from hydration:

Preparation of Aldehydes only
1. Rosenmund’s Reaction: Reduction of acid Chlorides with H2 and Pd/BaS04 catalyst in boiling xylene solution.

2. From Nitriles and esters: Stephen Reaction

Esters are reduced to aldehydes with DIBAL-H [diisobutyl- aluminium hydride]

3. From Aromatic hydrocarbons:
(a) With chromyl chloride (Cr02Cl2): Etard’s Reaction

(b) With Chromic Oxide (Cr03)

(c) By side-chain chlorination followed by hydrolysis

(d) By Gatterman-Koch Reaction

Methods of Preparation for Ketones:
1. From acid chlorides

2. From nitriles

3. From benzene: By Friedel-Crafts acylation:

→ Physical Properties:
1. Methanal is a gas at room temperature. Ethanol is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature.

2. B. Pts of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses due to weak molecular association in aldehydes and ketones due to dipole-dipole interactions. Their B.Pts. are lower than those of alcohols due to the absence of H-bonding.

The following compounds of molecular masses 58 and 60 are ranked in order of increasing boiling points.

3. The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions because they form H-bonds with water.

Solubility of aldehydes and ketones decreases rapidly on increasing the length of the alkyl group.

4. The lower aldehydes have a sharp pungent odour. As molecular mass increases, the odour becomes less pungent and more fragrant.

Chemical Reactions of Aldehydes and Ketones:
1. Nucleophilic addition Reactions: On the attack of a nucleophile on the carbonyl carbon, the hybridization of C changes from sp² to sp³.

Aldehydes are more susceptible to nucleophilic addition reactions than ketones due to steric and electronic factors.

The reactivity of aldehydes and ketones towards nucleophilic addition reactions is

The polarity of the carbonyl group is reduced in benzaldehyde due to resonance and hence it is less reactive than propanal.

1. Addition of Hydrogen cyanide (HCN):
HCN + OH- ⇌ -CN + H2O

2. Addition of Sodium bisulphite (NaHSO3):

3. Addition of Grignard’s reagent: They give 1°, 2°, 3° alcohols.

4. Addition of alcohols: Acetals/Ketals are formed

Ketones, under similar conditions, react with ethylene glycol to form cyclic products known as ethylene glycol ketals.

5. Addition of ammonia and its derivatives [Addition- Elimination].

Z = alkyl, aryl, OH, NH₂, C₆H₅NH, NHCONH₂ etc.


[Table: Some N-Substituted Derivatives of aldehydes and ketones

2, 4-DNP-derivatives are yellow, orange or red solids, useful for characterization of aldehydes and ketones.

2. Reduction:
1. Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄) as well as by catalytic hydrogenation.

2. Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction) or with hydrazine followed by heating with sodium or potassium hydroxide in a high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

3. Oxidation: Aldehydes differ from ketones in their oxidation reactions,

Due to the easy oxidation of aldehydes as compared to ketones, they can be distinguished from the following two steps:
1. Tollen’s test:
R—CHO + 2 [Ag(NH₃)₂]⁺ + 3 OH^– → RCOO^– + Ag (s) + 2 H₂O + 4 NH₃
On warming an aldehyde with freshly prepared ammonical AgNO₃ solution (Tollen’s reagent), a bright silver mirror is produced.

2. Fehling’s Test: On heating an aldehyde with Fehling reagent, a reddish-brown precipitate is obtained.
RCHO + 2 Cu²⁺ + 5 OH^– → RCOO^– + Cu₂O + 3 H₂O. (red-brown ppt.)

→ Oxidation of methyl ketones by haloform reaction: All those carboxyl compounds containing the – COCH₃ group respond to this test.

If NaOI is used, yellow ppt. of CHI₃ is formed which is used to detect the CH₃C = O group.

4. Reactions due to α-hydrogen atom: α-hydrogens in aldehydes and ketones are acidic in nature due to the strong electron-withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

1. Aldol Condensation: Aldehydes and ketones having at least one α-hydrogen undergo Aldol Condensation in the presence of dil. alkali to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones [ketol] respectively.


2. Cross Aldol Condensation: When aldol condensation is carried out between two different aldehydes and/or ketones, it is called Cross aldol condensation.

Ketones can also be used as one component in the cross aldol condensation.

5. Other Reactions:
1. Cannizzaro Reactions: Aldehydes that do not have an a- hydrogen atom, undergo self oxidation-reduction called Disproportionation reactions on treatment with concentrated alkali.

2. Electrophilic substitution reactions: Aromatic aldehydes and ketones undergo electrophilic substitution reactions at the ring and the carbonyl group acts as a deactivating and meta-directing group.

Uses of aldehydes and ketones

• Formalin (aqueous solution of formaldehyde) is used to preserve biological specimen and to prepare bakelite.
• Acetaldehyde is used to prepare acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.
• Benzaldehyde is used in perfumery and in dye industries.
• Acetone and ethyl methyl ketone are common industrial solvents.

II. Carboxylic Acids: Carbon compounds containing a carboxylic -COOH group are called Carboxylic acids.

→ Nomenclature and Structure of Carboxylic Group: In the IUPAC system last – e of alkanes is replaced with – oic acid. For naming compounds containing more than one carboxylic group, the ending – C of the alkane is retained.

Table: Names and Structures of Some Carboxylic Acids:

→ Structure of Carboxylic Group: Because of the possible resonance structure shown below, the carboxylic carbon is less electrophilic. Then carbonyl carbon and the bonds of the carboxylic carbon lie in the plane and are separated by 120°.

Methods of Preparation of Carboxylic Acids:
1. From Primary Alcohols and Aldehydes: Primary

2. From Alkyl benzenes:

3. From Nitriles and amides: By Hydrolysis

2. Due to extensive hydrogen bonding, carboxylic acid molecules get associated and thus have higher boiling points as compared to aldehydes, ketones and even alcohols of comparable molecular masses.

(In vapour state are in an aprotic solvent.)

3. Simple aliphatic carboxylic acids having up to four C atoms are miscible in water the formation of hydrogen bonds with water.

The solubility decrease with an increasing number of carbon atoms.

(Hydrogen bonding of RCOOH with H₂O)

Chemical Reaction:
Reactions involving cleavage of O-HBond

Acidic character Reactions with metals and alkalies:
2RCOOH + 2 Na → 2 RCOO^– Na⁺ + H₂ Sodium carboxylate
RCOOH + NaOH → RCOO^– Na⁺ + H₂O
RCOOH + NaHCO₃ → RCOO^– Na⁺ + H₂O + CO₂

Carboxylic acid dissociates in water to give resonance-stabilized carboxylate anion and the hydronium ion.

For the above reaction:

Where K_eq, is the equilibrium constant and K_a is the acid dissociation constant.
p^ka = – log k_a

The p^ka of hydrochloric acid is – 7.0, whereas p^ka of trifluoracetic acid (tine strongest organic acid), benzoic acid, and acetic acid are 0.23, 4.19 and 4.76 respectively.

Smaller the p^ka, the stronger the acid.
Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols [pka for phenols is ~ 10 and for ethanol ~ 16). Carboxylate anion is more resonance stabilized than phenoxide ion as the – ve charge is spread on two oxygen atoms rather than one in phenoxide ion.

Effect of Substituents on the acidity of carboxylic acids: Electron withdrawing groups increase the acidity of carboxylic acids by stabilizing the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. Conversely, electron-donating groups decrease the acidity by destabilizing the conjugate base.

The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO2 < CF3.

Thus the following acids are arranged in order of increasing acidity/based on p^ka values.

Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid, contrary to the decrease expected due to the resonance effect shown below:

This is because of the greater electronegativity of sp² hybridised carbon to which carboxyl carbon is attached. The presence of an electron-withdrawing group on the phenyl of the aromatic carboxylic acid increases their acidity while electron-donating groups decrease their acidity.

Reactions Involving Cleavage of C—OH Bond:
1. Formation of Anhydride

2. Esterification:

Mechanism of Esterification:

3. Reaction involving PCl₅ PCl₃ SOCl₂: Thionyl chloride (SOCl₂) is preferred as the other two products are gases.
RCOOH + PCl₅ → R COCl + POCl₃ + HCl
3RCOOH + PCl₃ → 3 ROCl + H3PO₃
RCOOH + SOCl₂ → RCOCl + SO₂ ↑ + HCl ↑.

4. Reactions with ammonia:

Reactions Involving: COOH Group:
1. Reduction:

2. Decarboxylation:

Kolbe’s Electrolysis

Substitution reactions in the hydrocarbon part:
1. Halogenation: Hell-Volhard-Zelinsky Reaction

2. Ring Substitution: —COOH group present in the ring is a deactivating group and meta-directing group. They do not, however, undergo Friedel-Craft reaction.

It is because the carboxylic group (— COOH) is a deactivating group and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxylic group.

Uses of Carboxylic acids: Methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Ethanoic acid is used as a solvent and as vinegar in the food industry. Hexanedioic acid is used in the manufacture of nylon-66. Esters of benzoic acid are used in perfumery. Sodium benzoate is used as a food preservative. Higher fatty acids are used for the manufacture of soaps and detergents.

By going through these CBSE Class 12 Chemistry Notes Chapter 11 Alcohols, Phenols and Ethers, students can recall all the concepts quickly.

Alcohols, Phenols and Ethers Notes Class 12 Chemistry Chapter 11

→ Alcohol contains one or more hydroxyl (OH) group (s) directly attached to a carbon atom (s).

→ A Phehol contains one or more hydroxyl group (OH) attached to a carbon atom (s) of the benzene ring.

→ An ether contains an alkoxy/aryloxy group (R-O/Ar-O) in place of the H atom of a hydrocarbon.

Classification:

• C₂H₅OH: Monhydric alcohol
• CH₂OH-CH₂OH: Dihydric alcohol
• HOH₂C-CHOH-CH₂OH: Trihydric alcohol
• 
  

1. Compounds containing C_sp³ -OH bond: In this class of alcohols, the -OH group is attached to an sp³ hybridized carbon atom of an alkyl group. They are further classified as follows:

→ Primary, secondary and tertiary alcohols: Here -the OH group is attached to a primary, secondary and tertiary carbon atom, respectively as shown below:

→ Allylic alcohols: In these alcohols, -OH group is attached to an sp³ hybridized carbon next to the carbon-carbon double bond, i.e., to an allylic carbon, e.g.,

→ Benzylic alcohols: In these alcohols, the -OH group is attached to an sp³-hybridized carbon next to an aromatic ring. For example

Allylic and benzylic alcohols may be primary, secondary or tertiary.

2. Compounds containing C_sp²-OH bond: These alcohols contain -OH group bonded to a carbon-carbon double bond.
Vinylic alcohols CH₂ = CH-OH

Ethers (A) Simple ethers/Symmetrical ethers (ROR)-
C₂H₅-O-C₂H₅ CH₃-O-CH₃.

(B) Mixed or Unsymmetrical ether (ROR’): R ≠ R’
C₂H-O-CH₃ C₆H₅-O-C₂H₅

Nomenclature:
(a) Alcohols: Common and I.U.P.A.C. names given below:

Table 11.1: Common and IUPAC Names of Some Alcohols:

Cyclic alcohols are named using the prefix cyclo and considering the-OH group attached to C-1.

(b) Phenols: The simplest hydroxy derivative of benzene is phenol, it is its common name and also an accepted IUPAC name. As the structure of phenol involves a benzene ring, in its substituted compounds the terms ortho (1, 2- disubstituted), meta (1, 3-disubstituted) and para (1, 4-disubstituted) are often used in the common names.

Dihydroxy derivatives of benzene are known as 1, 2-, 1, 3- and 1, 4-benzenediol.

(c) Ethers: Common names of others are derived from the names of alkyl/aryl groups written as separate words in alphabetical order and adding the word ‘ether’ at the end. For example, CH₃OC₂H₅ is ethyl methyl ether. If both the alkyl groups are the same, the prefix ‘di’ is added before the alkyl group. For example, C₂H₅OC₂H₅ is diethyl ether.

According to the IUPAC system of nomenclature, others are regarded as hydrocarbon derivatives in which a hydrogen atom is replaced by an -OR or -OAr group, where R and Ar represent alkyl and aryl groups, respectively. The larger (R) group being chosen as the parent hydrocarbon. The names of a few others are given as examples in the Table below:

Table 11.2: Common and IUPAC Names of Some Ethers:

Structures of Functional Groups-In alcohols, the oxygen of the -OH group is attached by a sigma (a) bond formed by the overlap of an sp³ hybridized orbital of carbon with an sp³ hybridized orbital of oxygen.

The figure below depicts the structural aspects of the methanol, phenol and methoxymethane.

Structures of methanol, phenol and methoxymethane.

The bond angle in alcohols is slightly less than the tetrahedral angle (109°28′) which is due to repulsion between the unshared electron pairs of oxygen.

Alcohols and Phenols:
Preparation of Alcohols:
1. From alkenes:
1. By acid-catalysed hydration

Mechanism:
→ Step I: Protonation of alkene to form carbocation by the electrophilic attack of H₃O⁺ ion,
H₂O + H⁺ → H₃O⁺

→ Step II: Nucleophilic attack of water on carbocation

→ Step III: Deprotonation to form an alcohol

3. By hydroboration-oxidation: Borane (BH₃) is an electrophile since it is electron-deficient. Addition product formed is oxidized to alcohols by H₂O₃  and aq. NaOH.

Addition of water proceeds against the Markovnikor rule.

2. From Carbonyl Compounds:
1. Reduction of aldehydes and Ketones:
(a) Hydrogen gets added in the presence of a catalyst (catalytical hydrogenation) like Pd, Pt, Ni (all finely. divided)
(b) By treating carbonyl compounds with sodium borohydride or lithium aluminium hydride (Li A1H4).

Aldehydes give primary alcohols whereas ketones yield secondary alcohols.

2. Reduction of carboxylic acids and esters:

3. From Gngnard reagents:


Preparation of Phenols:
1. From haloarenes

2. From benzene Suiphonic acid

3. From diazonium salts

4. From Cumene

→ Physical Properties of Alcohols and Phenols: The properties of alcohols and phenols are chiefly due to the hydroxyl group. The nature of alkyl and aryl groups simply modify their properties.

→ Boiling Points: The boiling points of alcohols and phenols increase With the increase in the number of carbon atoms (increase in van der Waals forces). In alcohols, the boiling points decrease with the increase of branching in the carbon chain (because of a decrease in van der Waals forces due to a decrease in surface area).

The -OH group is alcohols and phenols is involved in intermolecular hydrogen bonding as shown below:

The high boiling points of alcohols are mainly due to the presence of intermolecular hydrogen bonding in them which is lacking in ethers and hydrocarbons.

→ Solubility: Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules. The solubility decreases with an increase in the size of alkyl/aryl (hydrophobic) groups. Lower alcohols are miscible with water in all proportions.

→ Chemical Reactions: Alcohols react both with nucleophiles and electrophiles. The bond between O-H is broken when alcohols react as nucleophiles.
1.

2. The bond between C-O is broken when they react as electrophiles. Protonated alcohols react in this manner.

Based on the cleavage of O-H and C-O bonds, the reactions of alcohols and phenols may be divided into two groups.
(a) Reactions involving cleavage of O-H bond:
1. Acidity of alcohols and phenols

→ Reactions with metals:
2R-O-H + 2 Na → 2 RONa + H₂

→ Reaction with Aq. NaOH: Phenols react with aq. NaOH

→ Alcohols and Phenols are Bronsted acids i.e., they can donate a proton to a stronger base [: B]

→ The acidity of alcohols:

Due to the electron-releasing group (-CH₃, -C₂H₅) electron density on oxygen increases and the polarity of the O-H bond decreases. This decreases the acid strength.

Alcohols are, however, weaker acids than water.

Water is a better proton donor (i.e., stronger acid) than alcohol. Alkoxides (e.g. sodium ethoxide) is a better proton-acceptor than hydroxide ion, which suggests that alkoxides are stronger bases. (Sodium ethoxide is a stronger base than sod. hydroxide).

Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors.

→ The acidity of Phenols: The reactions of phenols with metals like Na, Al and NaOH indicate their acidic nature. The -OH group in phenols is directly attached to the sp2 hybridized carbon of benzene ring which acts as an electron-withdrawing group.

The reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water. Let us examine how a compound in which the hydroxyl group attached to an aromatic ring is more acidic than the one in which the hydroxy] group is attached to an alkyl group.

The ionisation of alcohol and a phenol takes place as follows:

Due to the higher electronegativity of sp² hybridised carbon of phenol to which -OH is attached, electron density decreases on oxygen. This increases the polarity of the O-H bond and results in an increase in ionisation of phenols than that of alcohols. Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ions, the charge is delocalised.

The delocalisation of negative charge (structures I-V) makes phenoxide ion more stable and favours the ionisation of phenol. Although there is also charge delocalisation in phenol its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion.

In substituted phenols, the presence of electron-withdrawing groups such as the nitro group enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion.

On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in a decrease in acid strength. Cresols, for example, are less acidic than phenol.

Table 11.3: pKa Values of Some Phenols and Ethanol:

From the above data, you will note that phenol is a million times more acidic than ethanol.

2. Esterification: Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.

The introduction of acetyl (CH₃ CO^–) group in alcohols or phenols is known as acetylation. Acetylation of salicylic acid produces aspirin which possesses analgesic, anti-inflammatory and antipyretic properties.

(b) Reactions involving cleavage of C-O bond in alcohols.
1. Reaction with HX:
ROH + HX → R-X + H₂O.

Luca’s Test distinguishes the three classes of alcohols (1°, 2° and 3°) on reaction with cone. HCl and ZnCl₂ [Luca’s reagent], 3° alcohols produce turbidity immediately with it. 2° alcohols do it after some time. 1° alcohol does not produce turbidity at room temperatures.

2. Reactions with phosphorus trihalides:
3R-OH + PCl₃ → 3R-Cl + H₃P0₃

3. Reaction with a protic acid: e.g., cone. H₃P0₄ or H₂SO₄ causes dehydration of alcohols producing alkenes.

Thus the relative ease of dehydration of alcohols follows the order Tertiary > Secondary > Primary

Mechanism of dehydration of alcohols:
→ Step I: Formation of protonated alcohol

→ Step II: Formation of the carbocation. It is the slow step and hence the rate-determining step.

→ Step III: Formation of ethene by loss of a proton.

4. Oxidation:

(PCC is pyridinium chlorochromate-a complex of chromium oxide with pyridine and HCl)

Tertiary alcohols do not undergo oxidation reactions.

5. Dehydrogenation with red hot copper: I° and 2c alcohol form aldehydes and ketones respectively. 3° alcohols undergo dehydration.

(c) Reactions of Phenols:
Following reactions are shown by phenols only.
1. Electrophilic aromatic substitution: The —OH group fused in the ring activates the ring towards electrophilic substitution directing the incoming group at ortho and para positions.
1. Nitration:

The o- and p-isomers can be separated by steam distillation, o-, Nitrophenol is steam volatile due to intramolecular H-bonding while p-nitrophenol (Higher B.Pt) is less volatile due to intermolecular H- bonding which causes the association molecules.

2. Halogenation:

Reaction with Bromine water is used as a test of phenol.

2. Koibe’s reaction:

3. Reimer-Tiemann reaction: Salicylaldehyde is formed.

4. Reaction with zinc dust: Benzene is formed.

5. Oxidation:

Some Commercially Important Alcohols:
1. Methanol (CH₃OH): It is also known as wood-spirit.

Properties: It is a colourless liquid. It boils at 337 K. It is highly poisonous in nature. Ingestion of even small quantities causes blindness and large quantities cause even death.

Uses:

• It is used as a solvent in paints, varnishes etc.
• It is chiefly used for making formaldehyde.

2. Ethanol (C₂H₅OH):
Commercial Preparation: By fermentation of molasses

Properties:

1. It is a colourless liquid with B.Pt 351 K.
2. Combined with CuSO₄ and pyridine, it is termed as denatured spirit

Uses:

• It is an excellent solvent.
• In the laboratory and hospitals for sterilisation of surgical instruments.

Ethers:
Preparation of Ethers:
1. By dehydration of alcohols:

It is a nucleophilic bimolecular reaction (SN2)

Step:


2. Willamson synthesis of ethers: It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers.

It involves the S_N² attack of an alkoxide ion on primary alkyl halide.

Physical Properties of ethers:

1. The C-O bonds in ethers are polar and thus ethers have a net dipole moment.
2. Their b. pts are comparable to those of alkanes with comparable molecular masses but lower than those of alcohols. It is due to the lack of H-bonding in ethers.
3. The miscibility of ethers with water resembles those of alcohols of the same molar mass. It is due to the fact that-ethers like alcohols can form H-bonds with water.
   

Chemical Reactions: Ethers are the least reactive of the functional groups.
1. Cleavage of C-O bond in ethers-lire the cleavage of C-O bond in ethers takes place under drastic conditions with an excess of hydrogen halides.

The order of reactivity is HI > HBr > HCl.

Mechanism:
→ Step I:

→ Step II:

With HI in excess and at a high temp., ethanol reacts with another molecule of HI and is converted to ethyl iodide.

→ Step III:

However, when one of the alkyl groups is tertiary, the halide formed is a tertiary halide.

It is because, in step-2 of the reaction, the departure of leaving group (HO- CH₃) creates a more stable carbocation [(CH₃)₃ C⁺] and the reaction follows the S_N¹ mechanism.

2. Electrophilic substitution: The alkoxy group (-OR) is ortho and para directing and activates the aromatic ring towards electrophilic substitution.

1. Halogenation: Phenylalkyl ethers undergo usual halogenation in the benzene ring, e.g., anisole undergoes bromination with bromine in ethanoic acid even in absence of iron (III) bromide catalyst. It is due to the activation of the benzene ring by the methoxy group. Para isomer is obtained in 90% yield.

2. Friedel Crafts reaction: Anisole undergoes Friedel Crafts reaction, i.e., the alkyl and acyl groups are introduced at ortho and para positions by reaction with an alkyl halide and acyl halide in the presence of anhydrous aluminium chloride (a Lewis acid) as a catalyst.


3. Nitration: Anisole reacts with a mixture of the cone. H₂SO₄ and HNO₃ to yield a mixture of ortho and para nitro anisole.

By going through these CBSE Class 12 Chemistry Notes Chapter 10 Haloalkanes and Haloarenes, students can recall all the concepts quickly.

Haloalkanes and Haloarenes Notes Class 12 Chemistry Chapter 10

In Haloalkanes X is attached to sp³ hybridized carbon atom, whereas it is attached to sp² hybridized carbon atom in the aryl group.

Classification:

(a) Alkyl halides or Haloalkanes (R-X) [sp³ C-X Bond]
General Formula: C_n H_2n-1X

(b) Allylic halides: Here halogen atom is bonded to an sp³-hybridized carbon atom next to C = C, i.e., to an allylic carbon.

(c) Benzylic halides: Halogen is bonded to sp³ carbon next to the aromatic ring.

Compounds containing sp² C-X Bond:



IUPAC name:

Common and IUPAC names of some halides:

Nature of C-X bond: Due to the difference in electronegativity of C and X, the C-X bond is polarised; carbon bears a partial positive charge whereas the halogen atom bears a partial negative charge.

Carbon-halogen bond length increases from C-F to C-I as the size of the halogen atom increases.

Methods of Preparation:
1. From alcohols

The order of reactivity of alcohols with a given haloacid is 3° > 2° > 1°.

2. From hydrocarbons:
1. By Free radical halogenation: It gives a complex mixture of isomeric mono and polyhaloalkanes which is difficult to separate.

2. By electrophilic Substitution: Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron (III) chloride.

The ortho and para isomers can be easily separated due to large differences in their melting points.

3. Sandmayer’s reaction:

4. From alkenes: (a) Addition of hydrogen halides

Addition to unsymmetric alkenes is as per Markovnikov’s Rule

(b) Addition of Halogens

5. Halogen Exchange: Finkelstein Reaction
R-X + Nal → R-I + NaX
X = Cl, Br

Swartz Reaction: This method is used to prepare alkyl fluorides by heating an alkyl chloride/bromide in the presence of AgF/Hg₂F₂.
H₃C-Br + AgF → H₃C-F + AgBr

Physical Properties:

1. Alkyl halides are colorless when pure. However, bromides and iodides develop color when exposed to light.
2. Melting & b.Pts: Lower members are gases at room temperature. Higher members are liquids or solids.

Due to the polar character of the C-X bond and higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in halogen derivatives. That is why boiling points of chlorides, bromides, and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass.

For the same alkyl group, the boiling points increase from RF to RI in the order RF < RCl < RBr < RI.
For isomeric haloalkenes, the b.pts decrease with an increase in branching (lesser the surface area)

B. pts of isomeric di-halogens are very nearly the same. However, para isomers are higher melting as compared to their ortho and meta isomers. It is due to the symmetry of para isomers that fits in the crystal lattice better as compared to ortho and meta isomers.

3. Density: Bromo, iodo, and poly-chloro derivatives of hydrocarbons are heavier than water. The density increases with an increase in the number of carbon atoms, halogen atoms, and atomic mass of halogens.

4. Solubility: The haloalkanes are only very slightly soluble in water. However, they tend to dissolve in organic solvents.

Chemical Reactions:
A. Reactions of haloalkanes:

1. Nucleophilic substitution reactions (S_N)
2. Elimination reactions
3. Reactions with metals.

1. Nucleophilic Substitution reactions: A nucleophile (Nu^– 🙂 reacts with haloalkane (the substrate) which has a polar C-X bond.



Such reactions in which a stronger nucleophile displaces a weaker nucleophile are called Nucleophilic Substitution (S_N) reactions and the halide ion which departs with its bonding pair of electrons is called the leaving group. The better the leaving group, the more facile is the nucleophile substitution reaction. It follows the order
I^– > Br > Cl^– > F^–
∴ The order of reactivity of haloalkanes follows the sequence Iodoalkanes > Bromoalkanes > chloroalkanes > fluoroalkanes.

Types of Nucleophilic Substitution reactions:

1. S_N² [Substitution, nucleophilic, bimolecular]
2. S_N¹ [Substitution, nucleophilic, unimolecular]

1. Substitution nucleophilic bimolecular (S_N²): The reaction between CH₃Cl and hydroxide ion to yield methanol and chloride ion follows second-order kinetics, i.e., the rate depends upon the concentration of both reactants.

rate of reaction ∝ [Base] [R-X]

Since the rate of the reaction depends upon the concentration of both the reactants, it is a bimolecular nucleophilic displacement reaction.

There occurs a complete stereochemical inversion of the configuration. The order of reactivity of the alkyl halides is Primary halide > Secondary halide > Tertiary halide.

In the S_N² reaction, the attack of the nucleophile (OH^– above) occurs from the backside, and the halide ion leaves from the front side. This inversion of configuration is called Walden Inversion. As far as the ease of departure of halide ion is concerned, the order of reactivity is RI > RBr > RCl > RF.

2. Substitution, nucleophilic, unimolecular (S_N¹) – S_N¹ reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc). The reaction between tert-butyl bromide and hydroxide ion yields tert-butyl alcohols and follows first-order kinetics.

This reaction is independent of the concentration of the base. The rate law suggests the reaction proceeds in two steps.

This step is slow and hence is the rate-detaining step.

This step, being fast, does not affect the rate of reaction. If the alkyl halide is optically active, then the product is a racemic mixture.

Allylic and benzylic halides show high reactivity towards SN1 reaction

The carbocation gets stabilized through resonance as shown below:

For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both mechanisms.
R-I > R-Br > R-Cl > > R-F.

Stereochemical aspects of nucleophilic substitution reactions: An S_N² reaction proceeds with complete stereochemical inversion of configuration while an S_N¹ reaction proceeds with racemization.

Optical Activity: Certain compounds exhibit the property of rotating the plane polarised light when it passed through their solutions. Such compounds are called Optically Active compounds arid this phenomenon is called Optical Activity.

If the compound rotates the plane-polarised light to the right, i.e., in a clockwise direction, it is called dextro-rotatory or the d-form and is indicated by placing a positive (+) sign before the degree of rotation. If the light is rotated towards the left (anticlockwise), the compound is said to be laevorotatory or the /-form and a negative sign (-) is placed before the degree of rotation. Such (+) and (-) isomers of a compound are called Optical Isomers and this phenomenon is termed Optical Isomerism.

All the physical properties of compounds showing optical activity are the same like refractive index solubility, density, m.pts, b.pts, etc. Even the extent of rotation is the same. They differ from each other only in the direction of rotation.
If all the substituents attached to the C atom are different, such a carbon atom is called asymmetric carbon or stereocentre.

The resulting molecule would lack symmetry and is referred to as asymmetric or ‘dissymmetric molecule. This asymmetry of the molecule is responsible for the optical activity in such organic compounds. Such molecules are non-superimposable on their mirror image (as the left hand is non-superimposable on the right hand) and are said to be Chiral. This property of non-super imposibility of the mirror image on the object is called Chirality. The objects which are superimposable on their mirror image are called Achiral.

Butan-2-ol has 4 different groups attached to the tetrahedral carbon atom and is Chiral. The mirror image of Butan-2-ol non-superimposable onbutan-2-ol

Other chiral molecules are Bromochloroiodimethane (BrCl CHI), 2-chlorobutanol, 2, 3-dihydroxypropanal (OHC- CHOH-CH₂OH), lactic acid (CH₃CH(OH)COOH). The stereoisomers related to each other as non-superimposable are also called Enatiomers and the concept Enantiomerism.

A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be canceled by the rotation due to the other isomer. Such a mixture is called Racemic Mixture or Racemic Modification. It is represented by prefixing dl or (±) before the name, e.g., (±) butan-2-ol. The process of conversion of enantiomer into a racemic mixture is known as Racemisation.

Retention: Retention of configuration is the preservation of the integrity of the spatial arrangement of bonds to an asymmetric center during a chemical reaction or transformation. It is also the configurational correlation when a chemical species XCabc is converted into the chemical species YCabc having the same relative configuration.

Inversion, retention, and racemization: These are three possibilities for a reaction to occur at an asymmetric carbon atom.

Consider the replacement of a group X and Y in the following reaction:

If (A) is the only compound obtained, the process is called retention of configuration.
If (B) is the only compound obtained, the process is called inversion of configuration.
If a 50: 50 mixture of (A) and (B) is obtained, the process is called racemization and the product is optically inactive.

Thus during an S_N² reaction involving an optically active alkyl halide, the reactant undergoes inversion of configuration.

In the case of optically active alkyl halides, S_N¹ reactions are accompanied by racemization. Consider hydrolysis of optically active 2-romobutane, which results in the formation of(±) butan-2-oI.

Step I:


2. Elimination Reactions: When a haloalkane with a β-hydrogen atom is heated with an alcoholic solution of potassium hydroxide, there is an elimination of hydrogen atom from β-carbon and a halogen atom from the a-carbon atom.

An alkene is formed as a result. Since the β-hydrogen atom is involved in elimination, it is often called β-elimination.

If there is the possibility of the formation of more than one alkene due to the availability of more than one β-hydrogen atom, usually one alkene is formed as the major product. These form part of a pattern first observed by Russian Chemist Alexander Zaitsev (also pronounced as Saytzeff) who in 1875 formulated a rule which can be summarised as in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.” Thus, 2-bromopentane gives pent-3-ene as the major product.

An alkyl halide with β-hydrogen atoms when reacted with a base or a nucleophile has two competing routes: substitution (S_N¹ and S_N²) and elimination. The route to be taken up depends upon the nature of alkyl halide, strength and size of base/nucleophile, and reaction conditions.

Thus, a bulky nucleophile abstracts a proton rather than approaches a tetravalent C atom (steric hindrance). Similarly, a primary alkyl halide will prefer an S_N² reaction, a secondary halide S_N² or elimination depending upon the strength of base/nucleophile, and a tertiary halide: S_N¹ or elimination depending upon the stability of carbocation or the more substituted alkene.

3. Reaction With Metals: Most organic chlorides bromides and iodides react with certain metals to give compounds containing carbon-metal bonds. Such compounds are known as organometallic compounds.

In the Grignard reagent, the C-Mg bond is covalent but highly polar, the MgX bond is essentially ionic.

The Grignard reagent is highly reactive. Even H₂O reacts with it.

Wurtz Reaction: Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the reaction. This reaction is known as the Wurtz reaction.

Reactions of Haloarenes:
1. Nucleophilic Substitution: Alkyl halides are extremely dull/ loss reactive towards S_N reactions due to the following reasons.
1. Resonance effect: In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the ring as given below:

C-Cl bond a partial double bond character due to one. As a result difficult to break the C-X bond and therefore less reactive towards S_N¹
2. Different hybridization of Carbon atom in C-X bond: In haloalkanes, the C is sp³ hybridized while in haloarenes, the carbon attached to has is sp² hybridized.

The sp² hybridized carbon with \(\frac{1}{3}\) s-character is more electronegative and can hold the electron pair of C-X bond more tightly than sp3-hybridized carbon in haloalkane ‘With s-character. Thus

The C-Cl bond length in haloalkane is 177 pm while in haloarene it is 169 pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards SN reactions.

3. Instability of phenyl cation: In the case of haloarenes, the phenyl cation formed as a result of self-ionization will not be stabilized by resonance and, therefore, the S_N¹ mechanism is ruled out.

4. Because of the possible repulsion, it is less likely for the electron-rich nucleophile to approach electron-rich arenes.
Replacement by hydroxyl group:

The presence of an electron-withdrawing (-NO₂) at ortho and para position increases the reactivity of haloarenes.
The effect is pronounced when the (-NO₂) group is introduced at ortho and para positions. However, no effect on the reactivity of haloarenes is observed by the presence of an electron-withdrawing group at meta-position. Mechanism of the reaction is as depicted

The effect is pronounced when (—NO₂ the) group is introduced at ortho and para positions. However, no effect on the reactivity of haloarenes is observed by the presence of an electron-withdrawing group at the meta position. The mechanism of the reaction is as depicted:



The presence of a nitro group at ortho- and para- position withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene.

The carbanion thus formed is stabilized through resonance. The negative charge appeared at ortho- and para- positions with respect to the halogen substituent is stabilized by -NO₂ group while in the case of mcfa-nitrobenzene, none of the resonating structures bear the negative charge on carbon atom bearing the -NO₂ group.

Therefore, the presence of a nitro group of meta-position does not stabilize the negative charge and no effect on reactivity is observed by the presence of the -NO₂ group of meta-position.

2. Electrophilic Substitution Reactions: Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation, and Friedel-Crafts reactions. Halogen atom besides being slightly deactivating is o, p-directing, therefore, further 1 substitution occurs at ortho- and para-positions with respect to the halogen atom.

The o, p-directing; therefore, further substitution occurs at ortho- and para-positions with respect to the halogen atom. The o, p-directing influence of halogen atom can be easily understood if we consider the resonating structures of halobenzene as shown:

Due to resonance, the electron density increases more at ortho- and para-positions than at mcia-positions. Further, the halogen atom because of its-I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.
1. Halogenation:

2. Nitration:

3. Sulphonation:

4. Friedel-Crafts reaction:


3. Reaction with metal:
Wurtz-Fitting Reaction: It is between an alkyl and aryl halide.

→ Polyhalogen Compounds: Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen compounds.

→ Dichloromethane (Methylene chloride CH₂Cl₃: It is widely used as a solvent in paint remover, as a propellant in aerosols, and as a process solvent in the manufacturing of drugs. It harms the human central nervous system.

→ Trichloromethane (Chloroform CHCl₃): It is used as a solvent for fats, alkaloids, iodine, waxes, rubbers, plastics, etc. It was used as a general anesthetic in surgery.

Chloroform is slowly oxidized to carbonyl chloride (phosgene) by air in the presence of light. It is extremely poisonous in nature.

It is therefore stored in colored bottles to cut off light and in well- stoppered fully filled bottles to cut off air.

→ Triiodomethane (Iodoform CHI3₃): It was used as an antiseptic, but the antiseptic properties are due to the liberation of free iodine and not due to the iodoform itself.

→ Tetrachloromethane (Carbon Tetrachloride CCl₄): It is used for the synthesis of chlorofluorocarbons and as a solvent. There is some evidence that exposure to CCl₄ causes liver cancer in humans. It causes dizziness, nausea, and vomiting which can cause permanent damage to nerve cells. The chemical may irritate the eyes on contact. It depletes the ozone layer when released into the air.

→ Freons: Chlorofluorocarbon compounds of methane and ethane are collectively known as freons. They are extremely stable, unreactive, non-toxic, non-corrosive, and easily liquefiable gases, Freon 12 (CCl₂F₂) is one of the most common freons in industrial use. Most freon, even that used in refrigeration, eventually makes its way into the atmosphere where it diffuses into the stratosphere where it is able to imitate radical change reactions that can upset the natural ozone balance.

→ p, p’-Dichlorodiphenyltrichloroethane (DDT): The use of DDT was effectively used against the mosquito that spreads malaria and lice that carry typhus. Many species of insects developed resistance to DDT, and DDT was also discovered to have a high degree of toxicity towards fish. The chemical stability of DDT and its fat solubility compounded the problem. The use of DDT was banned in the USA in 1973, although it is still in use in some parts of the world. Its chemical formula is

Many out of these polyhalogen compounds cannot be easily decomposed and cause of depletion of the ozone layer and are proving environmental hazards.