|Chapter 10: Conjugation in Alkadienes and Allylic Systems|
Radical Halogenation of Allylic systems
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.
|© Dr. Ian Hunt, Department of Chemistry, University of Calgary|