Propylbenzene, $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_3$, is treated with $\mathrm{Br}{}_2$ and light. The major monobromo product is

$\mathrm{Br}\cdot$ is highly selective and abstracts the benzylic H, because the benzylic radical is stabilised by resonance with the ring. This radical takes Br from $\mathrm{Br}{}_2$ to give $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{CHBrCH}{}_2\mathrm{CH}{}_3$. Ring bromination needs a Lewis acid, not light.