Radical Anti-Markovnikov Addition to Alkenes: Overview

Peroxides initiate a radical reaction where the bromine radical adds first to the less-substituted carbon of the alkene. This generates a more stable tertiary radical intermediate, which is then attacked by hydrogen to form the final alkyl bromide product. The peroxide effect explains why bromine attaches to the less-substituted position rather than the more-substituted one.

Radical stability determines which carbon the bromine radical attacks. A more stable tertiary radical intermediate forms when bromine adds to the less-substituted carbon. This stability preference drives the regioselective outcome, directing the bromine away from the more-substituted position and ensuring anti-Markovnikov regioselectivity in the overall reaction.

The large bromine radical preferentially adds to the less-hindered, less-substituted carbon to minimize van der Waals repulsions. This steric preference creates a more stable radical intermediate and reinforces the anti-Markovnikov regioselectivity observed in the reaction, complementing the radical stability effect.

No, the addition is not stereoselective. When a new chiral center forms, the bromine radical can attack from either face of the alkene, producing a racemic mixture of enantiomers. Both stereoisomers form in equal amounts because the radical intermediate is planar and equally accessible from both sides.

The peroxide-promoted anti-Markovnikov addition is unique to hydrogen bromide. Other hydrogen halides do not exhibit this reactivity pattern under peroxide conditions. This selectivity reflects the specific reactivity of the bromine radical and the thermodynamic favorability of the reaction pathway with HBr.

The product is an alkyl bromide where the bromine is attached to the less-substituted carbon of the original alkene. If a new chiral center is created, the product exists as a racemic mixture of enantiomers due to the non-stereoselective nature of the radical addition process.

Peroxides and hydroperoxides decompose to generate bromine radicals that initiate the reaction. These radicals then add to the alkene's less-substituted carbon, creating a stable radical intermediate. The peroxide acts as a radical initiator, enabling the characteristic anti-Markovnikov pathway that does not occur under normal ionic conditions.