Free Radical Addition Reactions

Contents

•       Anti Markovnikov’s rule

•       Peroxide effect

•       Mechanism

Learning Objectives

At the end of this lecture, student will be able to

•       State anti Markovnikov’s rule

•       Explain  mechanism involved in the peroxide effect

Radical addition of HBr to alkenes

•       Addition of hydrogen halides to alkenes was first studied systematically in the 1930s

•       Chemists observed that the addition of HBr sometimes gives Markovnikov addition, and

•       Sometimes gives non-Markovnikov addition

•       Puzzle was solved in 1933 when it was discovered that non-Markovnikov products are observed only in the presence of peroxides or other sources of radicals

•       In the absence of radicals, addition of HBr gives only the expected Markovnikov product

Radical addition of HBr to alkenes:  Mechanism

•       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 an alcohol and a bromine radical

•       Chain Propagation: A bromine radical adds to the C=C regioselectively to give the more stable carbon radical

•       Carbon radical reacts with a molecule of HBr to give the bromoalkane and generate a new bromine radical

•       Chain Termination: Combination of a carbon radical with a bromine radical, and the combination of two bromine radicals

Summary

•       Addition of HBr in the presence of peroxide – anti Markovnikov addition

•       Anti Markovnikov’s addition – follows free radical addition mechanism

•       It involves chain initiation, chain propagation and chain termination steps