Chapter 11: Arenes and Aromaticity

Reaction of Alkyl Benzenes with Halogens

Reaction type: Radical Substitution

Summary

• When treated with Br₂ or Cl₂ radical substitution of benzylic-H generates the benzyl halide and HX.
• The benzyl radical is quite stable so bromination will often by selective for the benzyl position.
• Note that there is no substitution of the benzene ring under these conditions (for this, see Chapter 12)
• Only chlorination and bromination are useful in the laboratory.
• NBS is an alternate source for bromine.
• Reaction proceeds via a radical chain mechanism similar to that of alkanes or allylic systems

N-Bromosuccinimide

Related reactions

• Radical Halogenation of Alkanes
• Radical Halogenation of Allylic systems

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.
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.
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.

© Dr. Ian Hunt, Department of Chemistry