18.7: Reactions at the Benzylic Position: Halogenation
Benzylic halogenation takes place under conditions that favor radical reactions such as heat, light, or a free radical initiator like peroxide.
The reaction of toluene with an excess of chlorine can produce multiple benzylic chlorinations. However, the reaction of N-bromosuccinimide or NBS with toluene in the presence of a peroxide forms benzyl bromide. Halogenation of larger alkyl side chains are highly regioselective and occur primarily at the benzylic position. Bromination of ethylbenzene at the benzylic position solely gives a monobromo organic product. Whereas chlorination of ethylbenzene gives 1-chloro-1-phenylethane as the major product in the ratio of 9:1. The regioselectivity of halogenation reaction can be explained by the resonance stabilization of the benzylic radical intermediate. Benzylic halogenation is important because halogen substituted at the benzylic position can further be replaced by a different group.