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.
20.23:
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.