Ethanamine reacts with benzenesulphonyl chloride to give N-ethylbenzenesulphonyl amide, which is soluble in alkali. Which of the following is the reason for this solubility?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The hydrogen attached to nitrogen is strongly acidic due to the strong electron withdrawing sulphonyl group.
In the sulphonamide from a primary amine, nitrogen still carries one hydrogen atom.
\( \displaystyle \ce{C6H5SO2Cl + H2N-C2H5 -> C6H5SO2NH-C2H5 + HCl} \)
The C6H5SO2– group pulls electrons strongly away from nitrogen, so this N–H hydrogen is acidic. The alkali removes it as a proton and the sulphonamide goes into solution as its salt:
\( \displaystyle \ce{C6H5SO2NHC2H5 + NaOH -> [C6H5SO2NC2H5]^-Na^+ + H2O} \)
The sulphonamide of a secondary amine has no H on nitrogen and stays insoluble; this difference is used to tell the amines apart.
Why the other options are incorrect
Option (1) The ethyl group releases electrons to nitrogen by the +I effect, so the sulphonamide is a stronger base than ethanamine. — Incorrect. A base does not dissolve in an alkali. The sulphonyl group in fact makes the nitrogen acidic, not more basic.
Option (2) The sulphonamide forms strong hydrogen bonds with water through the oxygen atoms of the sulphonyl group. — Incorrect. The solubility is in alkali, not in water; it comes from salt formation, not from hydrogen bonding. The sulphonamide of a secondary amine also has these oxygen atoms but is insoluble in alkali.
Option (3) The sulphonyl group is removed by the alkali, and the free ethanamine dissolves. — Incorrect. The sulphonamide is not broken down by the alkali; it dissolves as a whole, by losing the acidic hydrogen.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “(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.”