JEE Main · 2021 · 24 Feb 2021 · Shift 2 · Compounds Containing Nitrogen

Coupling with $\beta$-naphthol needs an arenediazonium ion, and only a primary aromatic amine gives one: treatment with $\mathrm{NaN}{\mathrm{O}}_2/HCl$ at $273$-$278$ K converts $\mathrm{Ar}-\mathrm{N}{\mathrm{H}}_2$ into $\mathrm{Ar}-{\mathrm{N}}_2^{+}$, which is stabilised by delocalisation into the ring. A tertiary amine such as N,N-dimethylaniline has no hydrogen on nitrogen and gives a $p$-nitroso compound instead, while a secondary amine such as N-methylaniline gives an N-nitrosoamine. Benzylamine is a primary amine but an aliphatic one, and its diazonium ion loses nitrogen immediately to give a carbocation. Aniline alone forms a stable benzenediazonium ion, which attacks the carbon next to the hydroxyl group of $\beta$-naphthol in alkali to give an orange azo dye, so the compound required is aniline.