JEE Main · 2025 · 08 Apr 2025 · Shift 2 · Hydrocarbons

Ethylbenzene is to be converted into p-bromostyrene. The first step is electrophilic ring bromination with $\mathrm{B}{\mathrm{r}}_2$ and iron as the Lewis acid catalyst, which installs bromine para to the ethyl group. The second step is free-radical chlorination with $\mathrm{C}{\mathrm{l}}_2$ under heat, which substitutes a hydrogen at the benzylic carbon. The third step is dehydrohalogenation with alcoholic KOH, which removes HCl to create the vinyl double bond. The required sequence is bromine with iron, then chlorine under heat, then alcoholic potassium hydroxide.