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Reaction of Alkyl Benzenes with Halogens

radical halogenation of benzylic CH

Reaction type: Radical Substitution

Summary

an alternate source of Br2 for radical bromination      N-Bromosuccinimide

Related reactions

RADICAL CHAIN MECHANISM FOR REACTION OF TOLUENE WITH Br2
Step 1 (Initiation)
Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.
initiation, formation of Br radical by homolytic cleavage
Step 2 (Propagation)
(a) A bromine radical abstracts a hydrogen to form HBr and a benzyl radical, then 
(b) The benzyl radical abstracts a bromine atom from another molecule of Br2 to form the benzyl bromide product and another bromine radical,  which can then itself undergo reaction 2(a) creating a cycle that can repeat.
propagation : benzyl radical formed and converted to product
Step 3 (Termination)
Various reactions between the possible pairs of radicals allow for the formation of Br2 or the product, benzyl bromide. These reactions remove radicals and do not perpetuate the cycle.
termination steps : radicals combine

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