Hydrogen bromide (HBr) typically follows Markovnikov's rule, which states that in the addition of a protic acid (such as HBr) to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom with fewer hydrogen atoms, and the halogen atom (in this case, bromine) adds to the carbon atom with more hydrogen atoms. This rule is based on the stability of the resulting carbocation intermediate formed during the addition reaction.
However, in the presence of peroxide (usually represented as "ROOR"), such as hydrogen peroxide (H2O2), the addition of HBr to an unsymmetrical alkene can proceed in a way that contradicts Markovnikov's rule. This is known as the "anti-Markovnikov" addition or the peroxide effect.
The peroxide effect occurs due to the presence of free radicals generated from the decomposition of peroxides. Peroxides act as radical initiators and produce highly reactive free radicals, such as hydroxyl radicals (•OH). These radicals can abstract a hydrogen atom from HBr, leading to the formation of a bromine radical (•Br) and a hydroxyl radical (•OH).
During the addition of HBr to an alkene in the presence of peroxide, the bromine radical (•Br) adds to the carbon atom with fewer hydrogen atoms, while the hydroxyl radical (•OH) adds to the carbon atom with more hydrogen atoms. This results in the formation of an alkyl radical intermediate on the carbon atom with more hydrogen atoms, and the bromine atom adds to this carbon, leading to the "anti-Markovnikov" product.
The presence of the peroxide-derived radicals alters the reaction mechanism, favoring the formation of the anti-Markovnikov product. This is because the alkyl radical intermediate formed on the more substituted carbon atom is more stable than a primary carbocation intermediate, which would be formed following Markovnikov's rule. The stability of the radical intermediate is attributed to the greater alkyl group substitution, which provides better delocalization and dispersal of the unpaired electron.
It's important to note that the peroxide effect is specific to the addition of HBr in the presence of peroxides and doesn't generally apply to other addition reactions. Other protic acids, such as HCl or H2SO4, would still follow Markovnikov's rule in the absence or presence of peroxides.