The H-Cl bond is stronger (430.5 k.J mol-1 ) than H-Hr bond (363.7 kJ mol-1 ), thus H-Cl is not cleaved by the free radical. The H-I bond is further weaker (296.8 kJ mol-3 ) than H-Cl bond. Thus H-I bond breaks easily hut iodine free radicals combine to form iodine molecules instead of adding to the double bond and hence peroxide effect is not observed in case of HCl and HI.