Chapter 6: Reactions of Alkenes: Addition Reactions

Reaction of Alkenes with HBr (radical)

Summary

• When treated with HBr, alkenes form alkyl bromides.
• However, under these conditions, the regioselectivity is anti Markovnikov

• Peroxides (or uv light) facilitate the formation of a bromine radical, RO^. + HBr -> ROH + Br^.
• Reaction proceeds via the more stable radical intermediate.
• Let's contrast this reaction with the non-radical alternative:

 

	Regular	Radical
Conditions	HBr (dark, N2 atmosphere)	HBr (peroxides, uv light)
Electrophile	H+	Br.
Intermediate	carbocation	radical
Regioselectivity	Markovnikov	Anti-Markovnikov

• The change in selectivity can be rationalised by releasing the reversal of the order in which the -H and -Br add since under "normal" conditions the electrophile is H+ and under radical conditions is Br^.

MECHANISM FOR RADICAL REACTION OF ALKENES WITH HBr
Step 1: An electrophilic bromine radical adds to the alkene to generate the 2o radical.
Step 2: Radical abstracts a H atom from another molecule of HBr, creating the alkyl bromide and another bromine radical.

© Dr. Ian Hunt, Department of Chemistry