Question:
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.
Answer:
Addition of HBr to Propene yields 2-bromopropane follows the Markownikov s rule. The negative part of the addendum molecule will attached to that C atom having the least number of hydrogen. It involves the formation of carbocations.
The mechanism can be shown as below.
H3C-CH= CH2 + H-Br → H3C-CH+ - CH3 + Br- → H3C-CH- CH3- Br
prop-1-ene 2-bromopropane
In the presence of benzoyl peroxide the addition of HBr to propene follows the anti Markownikovs rule.
This is applicable only to addition of HBr and not HI or HCl. This is also called as Kharasch effect. Free radicals are formed here. The free radicals are formed in two ways -I and II. Free radical I is more stable than II , hence the reaction proceeds in the formation of 1- Bromopropane.
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