Radical Anti-Markovnikov Addition to Alkenes: Mechanism

Peroxides or hydroperoxides initiate the radical chain reaction by undergoing homolytic cleavage of their weak O–O bond when heated, forming two alkoxy radicals. These alkoxy radicals then abstract a hydrogen atom from hydrogen bromide to generate bromine radicals, which start the propagation cycle and sustain the chain reaction.

Chain propagation occurs in two repeating steps. First, a bromine radical adds to the alkene double bond, forming a more stable tertiary alkyl radical. Second, this alkyl radical abstracts a hydrogen atom from hydrogen bromide, regenerating the bromine radical and forming the final product. These steps repeat continuously to sustain the reaction.

The bromine radical adds to the alkene double bond in a way that produces the more stable tertiary alkyl radical rather than a less stable primary alkyl radical. This regioselectivity occurs because the more substituted carbon can better stabilize the unpaired electron through hyperconjugation and inductive effects from surrounding alkyl groups.

Initiation consists of two steps. Heat triggers homolytic cleavage of the peroxide O–O bond, generating two alkoxy radicals. An alkoxy radical then abstracts a hydrogen atom from hydrogen bromide, producing a bromine radical that enters the propagation cycle and initiates the chain reaction.

Termination occurs when two bromine radicals couple together to form a stable bromine molecule. This coupling removes reactive radicals from the system, stopping the chain reaction and preventing further propagation cycles from continuing. The reaction ends when radical concentration becomes too low to sustain propagation.

Anti-Markovnikov addition occurs via a radical mechanism in the presence of peroxides, producing opposite regioselectivity compared to ionic Markovnikov addition. The bromine attaches to the less substituted carbon of the alkene, while hydrogen attaches to the more substituted carbon, yielding the anti-Markovnikov product.

The bromine radical is the key propagating species because it initiates each cycle by adding to the alkene double bond and is continuously regenerated at the end of each propagation cycle. This regeneration allows the bromine radical to repeat the two-step propagation process many times, sustaining the chain reaction efficiently.