Return to Contents Chapter 10: Conjugation in Alkadienes and Allylic Systems Ch 10 contents

Radical Halogenation of Allylic systems

Radical substitution of allyic system using X2
Reaction type: Radical Substitution

Summary

N-bromosuccinimide
Related reactions
RADICAL CHAIN MECHANISM  FOR ALLYLIC BROMINATION
Step 1 (Initiation)
Heat or uv light causes the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.
Step 2 (Propagation)
(a) A bromine radical abstracts a hydrogen to form HBr and an allyl radical, then 
(b) The allyl radical abstracts a bromine atom from another molecule of Br2 to form the allyl bromide product and another bromine radical,  which can then itself undergo reaction 2(a) creating a cycle that can repeat.
Step 3 (Termination)
Various reactions between the possible pairs of radicals allow for the formation of  Br2 or the product, allyl bromide. These reactions remove radicals and do not perpetuate the cycle.

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Dr. Ian Hunt, Department of Chemistry, University of Calgary