Question

why peroxide effect is observed in HBr only ?

Answer 1

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. ... With HBr, both the propagation steps are exothermic and HBr adds to the double bond by free- radical mechanism.

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