Question

What is peroxide effect ? Why is the peroxide effect observed only with HBr and not with HCl or HI ? Explain.

Answer 1

peroxide effect is anti markovnikov rule.In this the -ve part of hbr goes to that double bonded c atom which have more no. of hydrogen atom.
This is not obeserved in case of HCL and HI because the bond in these 2 is stronger then that of in hbr.

Answer 2

In presence of peroxide or sunlight, the addition of HBr to an unsymmetrical alkene takes place. This is opposite to Markovnikov's effect. This peroxide effect is also termed as anti-Markovnikov's effect.

The radical chain reactions are successful when the chain propagation step is exothermic .

HBr and HI does not shows peroxide effect because the endothermic chain propagation steps are reversible and will break the chain.

The H-I bond is weak so the first propagation step is endothermic. HI readily oxidized to ${ I }_{ 2 }$ in the peroxide effect.