Halogens can add to alkenes giving vicinal dihaloalkanes, but when this reaction is carried out at low halogen concentrations, radical mechanisms are favored. A widely used reagent in allylic brominations is NBS (N-bromosuccinimide).

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Halogenation mechanism

This process begins with the dissociation of the NBS in the presence of light. The bromo radicals cleave the allylic hydrogen, forming a resonance-stabilized radical that reacts with the NBS to give a mixture of products.
Stage 1. Initiation
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Stage 2. Propagation
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Stage 3. Termination.
When the reactants run out, the radicals begin to unite with each other, generating traces of products that are not considered in the overall reaction.