Alkene hydrobromination (anti-Markovnikov)
Abstract
Anti-Markovnikov hydrobromination of alkenes is a reaction in which hydrobromic acid (HBr) adds across the double bond of an alkene, yielding an alkyl bromide. To achieve anti-Markovnikov regioselectivity, the reaction must be carried out in the presence of a radical initiator, typically a peroxide, heat or light. This reaction is only effective with HBr. Radical additions of HCl and HI are not observed under standard conditions because the radical chain propagation steps are thermodynamically unfavorable.
Keywords
Alkene, Alkyl bromide, Anti-Markovnikov
Category
Alkene
Mechanism
The reaction proceeds through a radical chain mechanism consisting of initiation, propagation, and termination steps. Initiation: the peroxide undergoes homolytic cleavage upon heating or irradiation, generating alkoxy radicals. These radicals abstract a hydrogen atom from HBr to produce bromine radicals, which initiates the chain reaction. Propagation: the bromine radical adds to the alkene to generate the most stable carbon-centered radical. Consequently, bromine bonds to the less substituted carbon atom of the double bond, while the radical is formed on the more substituted carbon atom. This regioselectivity arises because tertiary radicals are more stable than secondary radicals, which are more stable than primary radicals. The resulting carbon radical then abstracts a hydrogen atom from another molecule of HBr, yielding the alkyl bromide product and regenerating a bromine radical, thereby propagating the chain. Termination: the chain reaction is terminated when two radicals recombine to form a non-radical product, consuming radicals without generating new ones.
