20.23 : Radical Anti-Markovnikov Addition to Alkenes: Mechanism

The anti-Markovnikov addition of hydrogen bromide to alkenes occurs in the presence of hydroperoxides or peroxides.

The mechanism proceeds via a radical chain reaction involving initiation, propagation, and termination steps.

The chain initiation occurs in two steps. The first step involves the homolytic cleavage of the weak O–O bond of peroxide triggered by heat, forming two alkoxy radicals.

The second initiation step involves the abstraction of a hydrogen atom by the alkoxy radical from the hydrogen bromide, giving a bromine radical.

Next is the chain propagation, which also proceeds in two steps. The first step is the addition of a bromine radical to the alkene double bond, forming a more stable tertiary alkyl radical.

The second propagation step involves the abstraction of a hydrogen atom by the alkyl radical from the hydrogen bromide, forming the final product and regenerating a bromine radical.

Repetitions of the two propagation steps lead to a chain reaction.

Finally, the coupling of two bromine radicals terminates the reaction.