Free Radical Addition Reactions

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

 

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