Chapter 8.8: Radical Addition of HBr to Alkenes

Question 1

Describe radical initiated non-Markovnikov addition of HBr to alkenes mechanism

Answer 1

o Step 1: Chain Initiation
 Homolytic cleavage of a dialkyl peroxide is induced by light or heat to give two alkoxy radicals. An alkoxy radical then reacts with HBr by hydrogen abstraction to give and alcohol and a bromine radical.
o Step 2: Chain Propagation
 A bromine radical adds to the carbon-carbon double bond regioselectively to give the more substituted (and more stable) carbon radical. The carbon radical, in turn, reacts with a molecule of HBr to give the bromoalkene and to generate a new bromine radical.
o Step 3: Chain Termination
 The most important chain termination steps are the combination of a carbon radical with a bromine radical and the combination of two bromine radicals. Each of these steps destroys one or both of the radical intermediates in the chain.