JEE Main · 2021 · 20 Jul 2021 · Shift 2 · Compounds Containing Nitrogen

Sulphanilic acid carries a primary aromatic amino group, which the nitrous acid generated from $\mathrm{NaN}{\mathrm{O}}_2$ and HCl at 273-278 K converts into the diazonium salt A. A diazonium ion is a weak electrophile, so it couples only with a strongly activated ring; the nitrogen lone pair of N,N-dimethylaniline supplies that activation, and attack occurs at the position para to the dimethylamino group, the activated site least hindered by the two N-methyl groups. Compound B is the azo dye 4-[(4-dimethylamino)phenylazo]benzenesulfonic acid, the free acid form of methyl orange, in which an $-\mathrm{N}=\mathrm{N}-$ bridge joins the sulfonic acid ring to the dimethylamino ring.