20.22 : Radical Anti-Markovnikov Addition to Alkenes: Overview

Radical addition of hydrogen bromide to alkenes using hydroperoxides or peroxides proceeds via an anti-Markovnikov addition, where the bromine is directed to the less substituted carbon, forming alkyl bromides.

The observed regioselectivity can be explained by considering the radical stability and steric effect.

Radical stability can be understood from the peroxide effect. The peroxide initiates a radical reaction, allowing the bromine radical to add first to the less-substituted carbon, generating a more stable tertiary radical intermediate.

Similar is the influence of the steric effect. A large bromine radical adds to the less-hindered carbon, minimizing the van der Waals repulsions and forming a more stable radical, thereby justifying the observed anti-Markovnikov addition.

Notably, the radical addition of hydrogen bromide for some alkenes produces a new chiral center.

However, the addition is not stereoselective, as a bromine radical can add from either face of the alkene, giving a racemic mixture of enantiomers.

Interestingly, the peroxide-promoted anti-Markovnikov addition is only observed with hydrogen bromide. Other hydrogen halides do not show this addition.