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

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Solution

Addition of HBr to propene yields 2-bromopropane. This is accordance with Markovnikov's rule. The reaction proceeds through ionic mechanism and involves formation of carbocation intermediate. Secondary carbocation is more stable than primary carbocation and is preferentially formed.
While in the presence of benzoyl peroxide, the same reaction yields 1 - bromopropane. This is in accordance with anti Makovnikov's rule. This happens in presence of peroxide and with HBr only. The reaction proceeds through free radical mechanism. The reaction involves formation of primary and secondary free radicals. Secondary free radical is more stable and preferentially formed.

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