Coupling Reactions

Coupling Reactions

Coupling reactions are electrophilic substitution reactions. Some examples are as follows :

General reaction

Examples:

Illustration 29. Give a test to distinguish between aniline and diethyl amine.

Solution: Aniline with benzene diazonium salt form a yellow dye but diethyl amine does not.

Analysis of Amines:

1. Hinsberg Test:

This test helps to distinguish 1°,2° and 3° amines. The Hinsberg’s reagent is benzene sulphonyl chloride.

Illustration 30. Hinsberg’s reagent is:

(A) Phenyl isocyanide (B) Benzene sulphonyl chloride

(C) p-toluene sulphonic acid (D) o-dichloro benzene

Solution: (B) is correct.

2. Carbylamine test (Isocyanide test):

This test is used to distinguish 1° amines from 2° and 3° amines This test is given by both 1° aliphatic and 1° aromatic amines.

Illustration 31. A positive carbylamine test is given by

(i) N, N−Dimethyl aniline (ii) 2, 4−Dimethyl aniline

(iii) N−Methyl ortho methylaniline (iv) p−methyl benzylamine

(A) 2, 4 (B) 2, 3

(C) 1, 2, 4 (D) 2, 3, 4

Solution: (A) is correct

3. Liebermann Nitroso reaction:

This test is given only by 2° amines (both aliphatic and aromatic). 2° amine is converted into nitroso amine by treating the amine with HNO2. On warming with phenol and conc. H2SO4, brown or red colour is formed at first, which changes to blue then to green. Colour changes to red on dilution and further to greenish blue or violet on treatment with alkali.

Illustration 33. How will you separate primary, secondary and tertiary amines by Hoffmann’s method (i.e., using diethyl oxalate)?

Solution. Hoffmann’s method: The amine mixture is heated with diethyl oxalate when primary amine forms a solid oxamide, secondary amine forms a liquid oxamic ester and tertiary amine does not react. The solid and liquid fractions are separated.