Question

Give an example for each describe the following reactions.

1. Hoffmann’s bromamide reaction

2. Coupling reaction

3. Gattermann reaction

Answer 1

1. Carbylamine reaction
1° amines (aromatic/aliphatic) On heating with chloroform and alcoholic solution of KOH gives isocyanides/carbylamines producing unpleasant smell.

2° and 3° (aromatic/aliphatic) do not give this reaction.

2. Diazotisation
Aromatic 1° amines into diazonium salts by action of HNO₂ and dil HCl at ice cold temperature as HNO₂ and product are unstable at higher temperature.

3. Hofmann’s bromamide reaction
Involves heating of an amide with bromine and caustic alkali to yield an amine with one carbon less than original amide.

4. Coupling reaction
Diazonium salt acts as electrophile and brings about substitution in electron rich aromatic ring such as phenol and amines. The result is formation of dye.
(i) with phenol – In basic medium (9 - 10 pH) at low temperature (273 - 278 K)

(ii) with amines - In acidic medium (ph = 4 - 5) at low temperature (273 - 278 K)

5. Ammonolysis - Alkyl halide (1°) react with alcoholic ammonia to form 1°, 2°, 3° amines along with 4°ammonium salts. It is carried out by heating alkyl halide with ale solution of NH3 in sealed tube at 370 K.
If ammonia is used in excess, 1° amine is major product

If alkyl halide is excess, it forms all 1°, 2°, 3°, 4° ammonium salts

6. Acetylation
Aliphatic amines:
Introduction of acetyl group (CH₃CO-) into any molecule.
1° and 2° amines react with acetyl chloride to form amides
But 3° amines has NO H on N, so do not react.

7. Gabriel phthalimide synthesis

Reaction between alkyl halide (R - X) and ammonia gives mixture, of 1°, 2° and 3° amines with tetraalkyl ammonium halide. Mixture is not separable.
To get pure 1° amine, Gabriel phthalimide synthesis is preferred.

2. The reaction of diazonium salt with phenols and aromatic amines to form azo compounds of the general formula Ar - N = N - Ar is called coupling reaction. The coupling with phenol takes place in mildly alkaline medium while with amines it occurs under faintly acidic condition.

3. Gattermann reaction

When benzene diazonium chloride reacts with Halogen acids in presence of Cu powder, aryl halide is formed as