How does benzenesulphonyl chloride (Hinsberg’s reagent) react with primary amines, and what is the significance of the resulting sulphonamide’s solubility in alkali?

Benzenesulfonyl chloride (Hinsberg’s reagent) reacts with primary amines to form sulphonamides. The reaction involves the substitution of the hydrogen atom on the nitrogen of the primary amine with a benzenesulfonyl group (-SO₂Ph). The resulting sulphonamide is insoluble in alkali due to the acidic nature of the sulfonyl group. However, this reaction is selective, as secondary and tertiary amines do not react with Hinsberg’s reagent under mild conditions. The insolubility of the sulphonamide in alkali serves as a useful test in amine classification, allowing differentiation between primary, secondary, and tertiary amines based on solubility behavior.