Side-chain halogenation at the benzylic position occurs through radical reactions in the presence of heat, light, or a radical initiator like peroxide.

For example, benzylic chlorination of toluene occurs in the presence of heat and light. An excess of chlorine drives multiple substitutions.

Similarly, benzylic bromination is achieved by using N-bromosuccinimide, or NBS, in the presence of a peroxide catalyst.

Halogenation of larger alkyl side chains is highly regioselective and occurs exclusively at the benzylic position.

For instance, bromination of ethylbenzene at the benzylic position gives a monobromo product. Similarly, chlorination of ethylbenzene gives 1-chloro-1-phenylethane as the primary product.

The high regioselectivity is dictated by the resonance stabilization of the benzylic radical intermediate.

Notably, halogenation helps introduce a functional group at the benzylic position, which could be exchanged for a different group.