Answer to Question #154598 in Organic Chemistry for Hitlesh Ajmera

Question

Why peroxide effect is for HBr and not for HCL and HI

Answer 1

H-Br forms free radicals by undergoing homolysis that's, it's mechanism of additional to a unsymmetrical alkene with peroxide presents is free radicals.

HCL on the other hand is a stable acid. The H-CL bond(430 kJ mol-1) is stronger compared to H-Br bond (378kJ mol^-1) and it is not broken symmetrically by the peroxide generated free radicals making HCL free radical addiction to alkenes impossible.

For HI, the H-I bond (297kJ mol^-1) is weaker compared to H-Br, it forms iodine free radicals by undergoing homolysis but the iodine free radicals have a propensity of combining amongst themselves to form iodine molecules instead of adding to the ethylenic bond. therefore the HI does not respond to the effect of peroxide.