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