Chapter 10: Conjugation in Alkadienes and Allylic Systems

Radical Halogenation of Allylic systems

Summary

• When treated with Br₂ or Cl₂, radical substitution of allylic C-H generates the allyl halide and HX.
• The process is very similar to that of alkanes (chapter 4 review)
• Reaction proceeds via an radical chain mechanism (review) which involves radical intermediates (review)
• The stability of the allylic radical (due to resonance) favours substitution at the allylic position.
• N-bromosuccinimide (NBS) can be used as an alternative source of Br₂.

• Halogenation of alkanes
• Halogenation of benzylic systems (see Chapter 11)

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