JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Compounds Containing Nitrogen

Direct bromination of aniline with $\mathrm{B}{\mathrm{r}}_2/FeB{\mathrm{r}}_3$ cannot be controlled, since the strongly activating $-\mathrm{N}{\mathrm{H}}_2$ group makes the ring so reactive that polybromination results. To obtain a single, ortho-brominated product selectively, the para position must first be blocked and the amino group's activating strength moderated.

Aniline is treated with concentrated ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$, which forms the anilinium salt and, on heating, undergoes sulfonation to install a $-\mathrm{S}{\mathrm{O}}_3\mathrm{H}$ group at the para position (sulfanilic acid), blocking that site. The amino group is then acetylated with $\mathrm{A}{\mathrm{c}}_2\mathrm{O}$ to $-\mathrm{NHCOC}{\mathrm{H}}_3$, moderating its activating power so that bromination with $\mathrm{B}{\mathrm{r}}_2$ proceeds cleanly ortho to the amide (the para position being occupied by $-\mathrm{S}{\mathrm{O}}_3\mathrm{H}$). Heating with ${\mathrm{H}}_2\mathrm{O}$ then removes the sulfonic acid group by reverse sulfonation, and $\mathrm{NaOH}$ hydrolyzes the acetamide back to the free amine.

The reagent sequence ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4,\mathrm{A}{\mathrm{c}}_2\mathrm{O},\mathrm{B}{\mathrm{r}}_2,{\mathrm{H}}_2\mathrm{O}(\Delta ),\mathrm{NaOH}$ delivers the ortho-bromoaniline as the major product.