Addition of HBr to unsymmetrical alkenes takes place contrary to the markovnikov's rule in presence of peroxide. Propylene on treatment with HBr/H2O2 provides 1-bromopropane. In the presence of peroxide (H2O2), addition of HBr to unsymmetrical alkenes like propylene takes place contrary to the Markovnikov's rule. This happens only with HBr but not with HCl and HI. Reaction of propylene with HBr/H2O2 is as follows