20.20 : 自由基取代：烯丙基溴化

Allylic halogenation of an alkene requires a low concentration of halogen and a high temperature.

To carry out allylic bromination at room temperature, N-bromosuccinimide, or NBS, is used as the reagent. It maintains a low bromine concentration and acts as a source of bromine radicals.

For example, propene reacts with NBS in the presence of light or peroxide to form 3-bromopropene.

The mechanism of allylic bromination involves initiation, propagation, and termination steps.

During the initiation step, NBS undergoes light-induced homolytic cleavage of the N–Br bond to form a bromine radical.

In the first propagation step, the bromine radical reacts with propene and abstracts allylic hydrogen, forming an allylic radical and HBr.

The generated HBr reacts with NBS to produce the bromine molecule required for further propagation of the reaction.

In the second propagation step, the bromine molecule reacts with the allylic radical to give allyl bromide and a bromine atom.

Finally, in the termination step, the different radicals couple with each other to give nonradical products, stopping further reaction.