19.29:
Amines to Sulfonamides: The Hinsberg Test
Sulfonyl chloride reacts with primary and secondary amines to produce corresponding substituted sulfonamides, but it does not react with tertiary amines as the amine has no N–H protons.
The electron-withdrawing sulfonyl group makes the sulfonamide N–H proton acidic enough to react with a base to produce a water-soluble salt.
To distinguish between the types of amines, respective amine mixtures in aqueous base are treated with benzenesulfonyl chloride—also known as the Hinsberg reagent. This is called the Hinsberg test.
A primary amine reacts with benzenesulfonyl chloride to form an N–substituted sulfonamide precipitate which dissolves in the base to generate a clear solution of a water-soluble salt. Further, the addition of an acid precipitates the N–substituted sulfonamide.
Similarly, secondary amines react with sulfonyl chloride to give water-insoluble N,N–disubstituted sulfonamides. Since this has no acidic proton, it stays insoluble in the added base.
Tertiary amines do not react with the Hinsberg reagent and so, no precipitate forms.
19.29:
Amines to Sulfonamides: The Hinsberg Test
The Hinsberg test is a method to identify primary, secondary and tertiary amines, named after its pioneer, Oscar Hinsberg. Here, amines are treated with benzenesulfonyl chloride, also known as the Hinsberg reagent, in the presence of an excess of aqueous base, followed by acidification. Based on the nature of the amines, different changes are observed.
Generally, a primary amine reacts with the Hinsberg reagent to produce an N-substituted benzenesulfonamide. The electron-withdrawing sulfonyl group makes the N–H proton acidic, enabling ready deprotonation in a base to form a water-soluble salt. Further acidification of the clear solution results in the precipitation of an N-substituted benzenesulfonamide.
Secondary amines generate an insoluble N,N-disubstituted sulfonamide upon reaction with benzenesulfonyl chloride. Due to the lack of an N–H proton, the compound does not react with the base. Tertiary amines do not react with the Hinsberg reagent. However, upon acidification, tertiary amines are protonated and become water-soluble.
