Which of the following amines, when treated with nitrous acid at 273–…

The correct answer is Option (2) → Aniline

• An azo dye forms only from a stable arenediazonium salt, which only an aromatic primary amine gives.

• Aniline reacts with nitrous acid (from \( \ce{NaNO2} \) and HCl) at 273–278 K to give benzenediazonium chloride, which is stable in the cold:

      \( \displaystyle \ce{C6H5NH2 + NaNO2 + 2HCl} \xrightarrow{273-278\,\mathrm{K}} \ce{C6H5N2^+Cl^- + NaCl + 2H2O} \)

• In alkaline medium, the diazonium ion couples with phenol at its para position (coupling reaction), giving p-hydroxyazobenzene, an orange azo dye:

      \( \displaystyle \ce{C6H5N2^+Cl^- + C6H5OH + OH^- -> C6H5-N=N-C6H4-OH + Cl^- + H2O} \)

Why the other options are incorrect

Option (1) Methylamine — Incorrect. A primary aliphatic amine: its diazonium salt is unstable and at once gives nitrogen gas and an alcohol, so nothing is left to couple with phenol.

Option (3) Ethylamine — Incorrect. Also a primary aliphatic amine; it liberates nitrogen and gives ethanol, not an azo compound.

Option (4) Propylamine — Incorrect. Also a primary aliphatic amine, with an unstable diazonium salt; no azo dye forms.

NCERT (2026-27 edition):
9.6 Chemical Reactions – “(a) Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts which being unstable, liberate nitrogen gas quantitatively and alcohols. …”
“(b) Aromatic amines react with nitrous acid at low temperatures (273-278 K) to form diazonium salts, a very important class of compounds used for synthesis of a variety of aromatic compounds …”

9.9 Chemical Reactions – “… The azo products obtained have an extended conjugate system having both the aromatic rings joined through the –N=N– bond. These compounds are often coloured and are used as dyes. Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene.”