Why do hydrogen chloride (HCl) and hydrogen iodide (HI) fail to exhibit the anti-Markovnikov peroxid — Hydrocarbons Chemistry Question

💡 Solution & Explanation

The peroxide effect requires a free radical chain mechanism. For the chain to propagate successfully, both propagation steps must be exothermic. For HCl, the homolytic cleavage of the $H-Cl$ bond to abstract a hydrogen is highly endothermic. For HI, while the $H-I$ bond is easily broken, the addition of the large, unreactive iodine radical ($I^\bullet$) to the alkene double bond is endothermic. Thus, only HBr works efficiently via this pathway.