Nitrous acid and Hinsberg's reagent

Nitrous acid is made in the reaction mixture from sodium nitrite and a mineral acid. The three classes of amine react with it differently.

Primary aliphatic amines give unstable aliphatic diazonium salts, which break down to an alcohol with quantitative release of nitrogen gas. Measuring that nitrogen is used to estimate amino acids and proteins.

Aromatic primary amines react with nitrous acid at 273–278 K to form diazonium salts, which are the starting point for many aromatic compounds.

Secondary and tertiary amines react with nitrous acid in other ways (not detailed in this unit).

Hinsberg's reagent is benzenesulphonyl chloride, C₆H₅SO₂Cl. A primary amine gives an N-alkylbenzenesulphonamide whose N–H is strongly acidic, because the sulphonyl group withdraws electrons, so the product dissolves in alkali.

A secondary amine gives an N,N-dialkylbenzenesulphonamide with no H on nitrogen, which is not acidic and does not dissolve in alkali. Tertiary amines do not react.

These differences are used to tell 1°, 2° and 3° amines apart and to separate a mixture of them. Nowadays p-toluenesulphonyl chloride is used in place of benzenesulphonyl chloride.