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Question
Describe a method for the identification of primary, secondary and tertiary amines. Also write chemical equations of the reactions involved.
Solution
Benzenesulphonyl chloride (C6H5SO2Cl), which is also known as Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides.
(i) 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 a strong electron-withdrawing sulphonyl group. Hence, it is soluble in alkali.
(ii) In the reaction with the secondary amine, N, N-diethylbenzenesulphonamide is formed.
Since N, N-diethylbenzene sulphonamide does not contain any hydrogen atoms attached to nitrogen atoms, it is not acidic and hence insoluble in alkali.
(iii) 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.
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