Tests for Amines

Hinsberg Test for Amines

A chemical test that is most commonly used for the identification of primary, secondary and tertiary amines is called the Hinsberg test.

The amine is shaken with benzenesulfonyl chloride, also known as the Hinsberg reagent, in the presence of an aqueous base, such as KOH or NaOH.

Observations
The reaction produces different results depending on the type of amine:
Primary amine: Forms a soluble sulfonamide salt that precipitates when acidified.
Secondary amine: Forms an insoluble sulfonamide
Tertiary amine: Does not react with the reagent, but becomes water-soluble when acidified

The reaction of benzenesulphonyl chloride with primary amine yields N-ethylbenzenesulphonyl amide.

The hydrogen attached to nitrogen in sulphonamide is strongly acidic due to the presence of strong electron withdrawing sulphonyl group. Hence, it is soluble in alkali.

In the reaction with secondary amine, N,N-diethylbenzenesulphonamide is formed.

Since N, N-diethylbenzene sulphonamide does not contain any hydrogen atom attached to nitrogen atom, it is not acidic and hence insoluble in alkali.

Tertiary amines do not react with benzenesulphonyl chloride. This property of amines reacting with benzenesulphonyl chloride in a different manner is used for the distinction of primary, secondary and tertiary amines and also for the separation of a mixture of amines. However, these days benzenesulphonyl chloride is replaced by p-toluenesulphonyl chloride.

The Hinsberg test is reliable, but not infallible. Speed of the reaction, temperature, concentration, and solubility of the amines must be considered.

Carbylamine Test for Amines

This test is also known as the isocyanide test. Aliphatic and aromatic primary amines when warmed with chloroform and an alcoholic solution of KOH, forms isocyanide or carbylamine having very unpleasant or foul smell.
R-NH₂ + CHCl₃ + 3KOH (alc.) → R-NC + 3KCl + 3H₂O
Secondary and tertiary amines do not give this test. This reaction is a test for primary amine. Therefore, this test can be used to distinguish primary amines from secondary and tertiary amines.

Libermann's Nitroso Reaction

This test is used to distinguish between 1°, 2°, 3° amines. Reagent used for this test is nitrous acid.
For primary amines, reaction with nitrous acid results to form aliphatic Diazonium salt. It is very unstable and decomposes rapidly to give out alcohol with release of nitrogen gas. This reaction is known as diazotization reaction.
For 2° aliphatic amines or aromatic amines, the reaction with nitrous acid is slow and in cold conditions.
(C₂H₅)₂NH + HONO → (C₂H₅)₂N−N=O + H₂O