JEE Main · 2023 · 15 Apr 2023 · Shift 1 · Compounds Containing Nitrogen

The target, 2-bromo-4-chlorobenzoic acid, is built from p-nitrotoluene, keeping the methyl group as the handle for the final carboxyl. The ring is first brominated with $\mathrm{B}{\mathrm{r}}_2/Fe$, the bromine entering ortho to the methyl group. The nitro group is then reduced with $Fe/{\mathrm{H}}^{+}$ to an amino group, which is diazotised with nitrous acid (HONO). Treating the diazonium salt with cuprous chloride replaces the diazonium group by chlorine through the Sandmeyer reaction. Finally $\mathrm{KMn}{\mathrm{O}}_4$ oxidises the methyl group to a carboxylic acid. The correct order of reagents is $\mathrm{B}{\mathrm{r}}_2/Fe$, $Fe/{\mathrm{H}}^{+}$, HONO, CuCl, then $\mathrm{KMn}{\mathrm{O}}_4$.