Amines are reactive in nature, mainly due to difference in electronegativity between Nitrogen and Hydrogen and, due to lone pair of electrons on Nitrogen. Amines behaves like nucleophiles and as good Lewis bases and there are various reactions to support this. Different degree of amines differ in their reactivity due to steric hindrance and electronic effects. Aromatic amines are less basic due to +R effect, hence have higher \(pK_b\) value than the aliphatic amines. Basic strength of amines also depend on stability of corresponding conjugate acid in aqueous medium.
'A' compound is treated with Hinsberg's reagent, subsequent addition of alkali to it gives a clear solution. Compound 'A' is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Primary amine
Hinsberg’s reagent is benzenesulphonyl chloride, \( \ce{C6H5SO2Cl} \). It reacts with primary and secondary amines to form sulphonamides, and the solubility of the sulphonamide in alkali tells the two apart.
A primary amine gives an N-alkylbenzenesulphonamide that still carries one hydrogen on nitrogen:
\( \ce{C6H5SO2Cl} \) + \( \ce{H2N-C2H5} \) → \( \ce{C6H5SO2NH-C2H5} \) + HCl
This N–H hydrogen is strongly acidic because of the electron-withdrawing sulphonyl group, so the sulphonamide dissolves in KOH/NaOH as its salt and a clear solution results:
\( \ce{C6H5SO2NH-C2H5 + KOH -> [C6H5SO2N-C2H5]^- K^+ + H2O} \)
Why the other options are incorrect
Option (2) Aldehyde — Incorrect. An aldehyde has no N–H group and does not form a sulphonamide with Hinsberg’s reagent; the test is meant for amines.
Option (3) Secondary amine — Incorrect. It gives an N,N-dialkylbenzenesulphonamide with no hydrogen on nitrogen; this product is not acidic and stays insoluble in alkali.
\( \ce{C6H5SO2Cl + (C2H5)2NH -> C6H5SO2N(C2H5)2 + HCl} \)
Option (4) Tertiary amine — Incorrect. It has no hydrogen on nitrogen and does not react with benzenesulphonyl chloride at all.
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.”