benzylic halogenation

Suppose you have an alkylbenzene, like toluene, and you want to put a bromine not on the ring but on the side-chain carbon next to the ring. Ring halogenation (with a Lewis acid) would put it in the wrong place. Benzylic halogenation does exactly what you want: it selectively replaces a hydrogen on the benzylic carbon — the carbon attached to the ring — with a halogen, leaving the ring itself alone.

It is a radical reaction, not an ionic one, so the conditions are the giveaway: you use a halogen with light (hv) or a radical initiator and heat, and crucially no Lewis-acid catalyst. The mechanism is a radical chain — a halogen radical abstracts a hydrogen from the benzylic carbon, producing a benzylic radical, which then grabs a halogen atom to give the product and regenerate the chain carrier. The reaction is selective for the benzylic position because the benzylic radical is unusually stable: its unpaired electron is delocalized by resonance into the aromatic ring, just as an allylic radical is stabilized by an adjacent double bond. For bromination, the reagent N-bromosuccinimide (NBS) is the standard choice, because it supplies a low, steady concentration of bromine that favors clean benzylic substitution.

Benzylic halogenation is a key handle in synthesis. The benzylic halide it makes (such as benzyl bromide, C6H5-CH2Br) has an excellent, reactive leaving-group position, so it readily undergoes nucleophilic substitution or elimination to install yet other groups, or serves as a starting point to build longer or branched chains off the ring. It nicely complements ring halogenation: same atoms, opposite site, decided entirely by whether you run ionic (Lewis acid) or radical (light/NBS) conditions.