Why peroxide effect is shown only by HBr and not by HCl or HI?
Or
Explain why Kharasch effect is shown by HBr only and not by HCl or HI.
The mechanism of addition of HBr to a unsymmetrical alkene (say propene) in the presence of peroxide is free radical i.e. "H-Br undergoes homolysis to form free radical.
HCl is a very stable acid H-Cl bond (430 kJ moH) is stronger than H-Br bond (378 kJ mol-1) and is not broken symmetrically by the free radicals generated by peroxide. Hence the free radical addition of HCl to alkenes is not possible.
In the case of HI, the H - I bond (297 kJ mo-1) is weaker than H-Br bond and undergoes homolysis readily to form iodine free radical. But iodine free radicals have greater tendency to combine amongst themselves to form iodine molecules rather than add to the ethylenic bond.
Hence HI does not respond to the peroxide effect.
