Alkenes belong to the group of unsaturated hydrocarbons, that is, one molecule of alkene contains at least one double bond. Due to the presence of pi electrons, they show addition reactions in which an electrophile attacks the carbon-carbon double bond to form the additional products. When HBr is added to unsymmetrical alkenes in the presence of peroxide, 1-bromopropane is formed contrary to 2-bromopropane (according to Markovnikov’s rule). This reaction is better known as anti-Markovnikov addition or Kharash effect after the name of M. S. Kharash who first observed it. This reaction is also known as Kharash effect or peroxide effect.
Anti Markovnikov addition is also an example of addition reaction of alkenes which is an exception to the Markovnikov’s rule. It is one of the few reactions following free radical mechanism in organic chemistry in place of electrophilic addition as suggested by Markovnikov. This reaction is observed only with HBr, not with HCl or HI.
Mechanism of Anti Markovnikov addition
Anti Markovnikov addition reaction is found to follow a free radical mechanism. The peroxide compound involved helps in the generation of free radicals. A general mechanism of anti-Markovnikov addition reaction is discussed below:
- Generation of free radical through homolytic cleavage of peroxide compound.
- Attack of generated free radical on hydrogen halide to form halide radical through hemolysis
- Attack of generated halide radical on alkene molecule to form alkyl radical through hemolysis.
- Attack of a generated alkyl radical on hydrogen halide to form alkyl halide through homolytic cleavage of hydrogen halide bond.
Fig: Anti Markovnikov addition mechanism
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