Which of the following reagent is used in Hinsberg test of amines?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(C_6H_5SO_2Cl\) in aq. \(NaOH\)
Hinsberg’s reagent is benzenesulphonyl chloride, \( \ce{C6H5SO2Cl} \). It reacts with primary and secondary amines to form sulphonamides, and the solubility of these in aqueous alkali tells the three classes apart:
Primary amine — gives an N-alkylbenzenesulphonamide whose N–H is acidic (made so by the strongly electron withdrawing sulphonyl group), so it dissolves in alkali:
\( \ce{C6H5SO2Cl} \) + \( \ce{H2N-C2H5} \) → \( \ce{C6H5SO2NHC2H5} \) + HCl
Secondary amine — gives an N,N-dialkylbenzenesulphonamide with no H on N; it is insoluble in alkali:
\( \ce{C6H5SO2Cl} \) + \( \ce{HN(C2H5)2} \) → \( \ce{C6H5SO2N(C2H5)2} \) + HCl
Tertiary amine — has no H on N and does not react.
Why the other options are incorrect
Option (1) \((CH_3CO)_2O\) and pyridine — Incorrect. This is acylation; primary and secondary amines both give amides, so it does not separate the classes. Pyridine only removes the acid formed.
Option (3) \(NaNO_2\) in aq. \(H_2SO_4\) — Incorrect. This generates nitrous acid; primary aliphatic amines give \( \ce{N2} \) gas and aromatic amines give diazonium salts. It is the nitrous acid test, not the Hinsberg test.
Option (4) \(CH_3I\) (excess) followed by \(AgOH\) — Incorrect. This is exhaustive methylation, which turns an amine into a quaternary ammonium hydroxide (used in Hofmann elimination).
NCERT (2026-27 edition):
Reaction with Arylsulphonyl Chloride – “Benzenesulphonyl chloride (C6H5SO2Cl), which is also known as Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides. (a) 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. (b) 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. (c) Tertiary amines do not react with benzenesulphonyl chloride.”