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.
20.22:
Radical Anti-Markovnikov Addition to Alkenes: Overview
The addition of hydrogen bromide to alkenes in the presence of hydroperoxides or peroxides proceeds via an anti-Markovnikov pathway and yields alkyl bromides.
The observed regioselectivity can be explained based on the radical stability and steric effect. From the radical stability perspective, adding hydrogen bromide in the presence of peroxide directs the bromine radical at the less substituted carbon via a more stable tertiary radical intermediate. Similarly, in the steric framework, the larger bromine radical is bonded to the less substituted carbon, which forms a more stable radical, minimizing the steric effect. Therefore, the formation of a stable radical intermediate determines the regioselective outcome, justifying the observed anti-Markovnikov orientation.
The radical addition of hydrogen bromide is not stereoselective. If a new chiral center is produced, the addition of bromine radical can occur from either face of the alkene. So, forming a new chiral center leads to a racemic mixture of enantiomeric products. Moreover, the anti-Markovnikov addition is only feasible with hydrogen bromide. The reaction is not observed with other hydrogen halides.