Why HBr shows exceptional behaviour when exothermic?
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Peroxide or kharash effect : In the presence of peroxide and light, addition of HBr to unsymmetrical alkenes occur contrary to Markovnikov's rule.
But HCl and HI do not show peroxide effect. Radical chain reactions are successful when propagation steps are exothermic. An endothermic propagation step is reversible and would break the chain. With HI, the first propagation step is endothermic, because H-I bond is weak. Moreover, HI is oxidized by peroxide to I2 under reaction conditions.
With HCl, the second propagation step is endothermic, because the H-Cl bond is strong.
With HBr, both the propagation steps are exothermic and HBr adds to the double bond by free- radical mechanism
But HCl and HI do not show peroxide effect. Radical chain reactions are successful when propagation steps are exothermic. An endothermic propagation step is reversible and would break the chain. With HI, the first propagation step is endothermic, because H-I bond is weak. Moreover, HI is oxidized by peroxide to I2 under reaction conditions.
With HCl, the second propagation step is endothermic, because the H-Cl bond is strong.
With HBr, both the propagation steps are exothermic and HBr adds to the double bond by free- radical mechanism
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