In the presence of peroxide addition of HBr to propene takes place according to anti Markovnikov’s rule but peroxide effect is not seen in the case of HCl and HI. Explain.
According to anti-Markovnikov’s rule Br add in propene in a less substitute side as shown in the below reaction.
In peroxide addition with HBr generate free radical.
It undergoes free radical mechanism.
Free radical mechanism proceed in three step; i.e. initiation, propagation and termination
In initiation step peroxide free radical attack on HBr, HCl , HI . Here HCl bond is stronger than HBr . So HCl bond cannot cleave by free radical.
In addition step Br free radical have sufficient energy to cleaved the C=C bond. But in case of I free radical it react with other I free radical forming I2molecule .
In case of HBr both process are exothermic and hence peroxide effect is observed.