Question

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.

Answer 1

Part-1 : Anti-markovnikov's rule

According to anti-markovnikov's rule addition of Br-take place in a less substituted side as shown in the below reaction:



Part-2 : Mechanism of the anti - markovnikov

In peroxide addtion with HBr there is generation of free radicals.It undergose free radical mechanism:

Mechanism

Free radical mechanisms proceed in three step $\to$ initiation, propagation and termination.





HCI being the very stable acid, H-CI Bond $430KJmo{l}^{-}$ is stronger than H-Br Bond $378KJmo{l}^{-1}$ and is not broken symmetrically when reacted with free radicals gererated by peroxide. Hence, the free radical addition of HCI to alkenes is not possible.

In the case of HI, the H-I Bond $\left(297KJmo{l}^{-1}\right)$ is weaker than H-Br bond and undergoes homolysis readily to form iodine free radical but the iodine free radical have greater tendency to get dimerize amongst themselves to form iodine molecule rather than add to the ethylenic bond.