18.3:
Reactions at the Benzylic Position: Halogenation
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.
18.3:
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.
