JEE Main · 2025 · 22 Jan 2025 · Shift 2 · Hydrocarbons

Photobromination proceeds by a free-radical chain mechanism, and the site of substitution is fixed by which C–H bond, once broken homolytically, gives the most stable carbon radical (stability order $3°>2°>1°$).

In sec-butylcyclohexane, the substituent carbon directly attached to the ring — bearing an ethyl group, a methyl group, and the ring carbon — is a tertiary carbon. Abstraction of its hydrogen by a bromine radical generates a tertiary radical, more stable than any secondary radical available elsewhere on the ring or side chain. Coupling of this radical with $\mathrm{B}{\mathrm{r}}_2$ in the propagation step places the bromine on that tertiary carbon.

The major product is the cyclohexane ring bearing a $-\mathrm{C}(\mathrm{C}{\mathrm{H}}_3)(\mathrm{Br})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$ group — bromination at the tertiary carbon of the sec-butyl substituent.