Free Radical Addition Reactions

Free Radical Addition Reactions


       Anti Markovnikov’s rule

       Peroxide effect


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


       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


Post a Comment