Question

Describe a method for the identification of primary, secondary and tertiary amines. Also write chemical equations of the reactions involved.

Answer 1

Primary, secondary and tertiary amines can be identified and distinguished by Hinsberg’s
test. In this test, the amines are allowed to react with Hinsberg’s reagent, benzenesulphonyl chloride (C₆H₅SO₂Cl). The three types of amines react differently with Hinsberg’s reagent. Therefore, they can be easily identified using Hinsberg’s reagent. Primary amines react with benzenesulphonyl chloride to form N-alkylbenzenesulphonyl amide which is soluble in alkali.

Due to the presence of a strong electron-withdrawing sulphonyl group in the
sulphonamide, the H−atom attached to nitrogen can be easily released as proton. So, it is
acidic and dissolves in alkali. Secondary amines react with Hinsberg’s reagent to give a sulphonamide which is insoluble in alkali.

There is no H−atom attached to the N-atom in the sulphonamide. Therefore, it is not acidic
and insoluble in alkali. On the other hand, tertiary amines do not react with Hinsberg’s reagent at all.