Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.

Explanation

Addition of HBr to propene is an ionic electrophilic addition reaction which follows Markovnikov rule. In this case, `H^(+)` adds to alkene to give a more stable `2^(@)` carbocation. This is rapidly attacked by nucleophile `Br^(-)` ion to give 2-bromopropane
`H- Br hArr H^(+)+ Br^(-)`
`CH_(3)-CH= CH_(2)+H^(+) overset("Slow") to underset("(more stable)")(underset(2^(@)-"Carbocation")(CH_(3)-CH^(+)- CH_(3)))`
`CH_(3)-CH^(+)-CH_(3)+ Br^(-) overset("fast")to CH_(3)-underset("2-Bromopropane")(underset(Br)underset(|)(CH-CH_(3)))`
In the presence of benzoyl peroxide, the reaction follows free radical addition. In this case, `overset(*)Br` free radical acts as electrophile which is obtained from the action of benzoyl peroxide on HBr.
`C_(6)H_(5)CO-O-O-COC_(6)H_(5) underset("Heat")overset("Homolytic fission") to 2C_(6)H_(5)COoverset(*)O to underset("Phenyl free radical")(2C_(6)overset(*)H_(5)+2CO_(2))`
`C_(6)overset(*)H_(5)+ HBr to C_(6)H_(6)+ underset("Brezene")(overset(*)Br)`
Br free radical attacks propene in such a way to generate a more stable `2^(@)` free radical. This free radical obtained rapidly abstracts a hydrogen atom of HBr to give 1-Bromopropane.
`CH_(3)-CH=CH_(2)+ overset(*)(Br) overset("Slow") to underset("2-Free radical (stable)")(CH_(3)-overset(*)(CH)-CH_(2)Br)`
`CH_(3)-overset(*)CH- CH_(2)Br+h-Br overset("fast")to CH_(3)CH_(2)CH_(2)Br+ overset(*)Br`