20.26: Radical Anti-Markovnikov Addition to Alkenes: Mechanism
The reaction of hydrogen bromide with alkenes in the presence of hydroperoxides or peroxides proceeds via anti-Markovnikov addition. The radical chain reaction comprises initiation, propagation, and termination steps.
The mechanism starts with chain initiation, which involves two steps. In the first chain initiation step, a weak peroxide bond is homolytically cleaved upon mild heating to form two alkoxy radicals. In the second initiation step, a hydrogen atom is abstracted by the alkoxy radical to form a bromine radical. The second part of the reaction is chain propagation. Here, the bromine radical is added to the alkene double bond to give a more stable tertiary alkyl radical. In the second propagation step, a hydrogen atom is abstracted by the alkyl radical to form the final product, followed by the regeneration of a bromine radical. These two propagation steps are repeated, which leads to a chain reaction. A final step is the combination of two bromine radicals to form a stable molecule, which terminates the reaction.
