Addition to HBr to propene (unsymmetrical alkene) follows Markovnikov's rule according to which the negative part of the addendum gets attached to that C atom which possesses lesser number of hydrogen atoms.
Mechanism: Electrophilic addition.
Step (i) It follows that proton H (an electrophile) from H-Br gets attached to C1 [due to + I effect exerted by–CH3 group, double bond between C1 and C2 undergoes electromeric effect].
Step (ii) In the (ii) Step.
The intermediate carbocation is attacked by Br– to form 2 Bromopropane
Mechanism: Peroxide effect proceeds via free radical mechanism as gives below:
(i)
(ii) C6H5 + H - Br \(\overset{Homolysis}{\rightarrow}\) C6H6 + Br
(iii) CH3 - CH = CH2 + Br \(\overset{Homolysis}{\rightarrow}\) CH3 - CH - CH2 - Br (more stable free radical)
(iv) CH3 - CH - CH2Br + H - Br \(\overset{Homolysis}{\rightarrow}\) CH3 - CH2CH2Br + Br
The secondary free radical obtained in (ii) in the above mechanism happens to be more stable than the primary one hence, explains the formation of 1-bromopropane.
