Cyclohexene is treated with N-bromosuccinimide (NBS) in $\mathrm{CCl}{}_4$ in the presence of light. The major organic product is

NBS supplies a very low concentration of $\mathrm{Br}{}_2$, so addition to the double bond is suppressed. Bromine radical abstracts an allylic hydrogen to give the resonance-stabilised cyclohex-2-enyl radical, which then picks up Br. The product is 3-bromocyclohexene.