JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Compounds Containing Nitrogen

$p$-Aminoazobenzene is made by coupling benzenediazonium chloride with aniline, which attacks through its para position, so every route must first reach a diazonium salt. Where nitrobenzene is reduced by tin and hydrochloric acid, aniline is obtained, nitrous acid diazotises it and the coupling follows. Where aniline itself is the starting material, nitrous acid diazotises it directly and the coupling with aniline in hydrochloric acid again succeeds. Sodium borohydride, however, is too mild to reduce an aromatic nitro group to an amine, so that sequence never produces a diazonium salt and no coupling can occur. Only the route in which nitrobenzene is treated with sodium borohydride fails to give $p$-aminoazobenzene.