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Master Organic Chemistry Reaction Guide

Free Radical Addition of HBr To Alkenes

Description: Treatment of an alkene with HBr in the presence of catalytic amounts of a radical initiator (such as peroxides) in addition to heat or light leads to addition of HBr to the alkene in anti-Markovnikov fashion (note that the Br adds to the less substituted side of the alkene and the H adds to the more substituted side).

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5 thoughts on “Free Radical Addition of HBr To Alkenes

  1. Why doesn’t the Br radical abstract the allylic hydrogen as it does in the allylic bromination mechanism. In both mechanisms, you have a step in which a Br radical reacts with an alkene, but they do different things. Please explain the difference.

    1. It probably happens to some extent, but the reaction is a cul-de-sac that doesn’t result in a brominated product.
      The reason is that once you form that allylic radical, what can it do?
      – there is no source of Br2 that would result in bromination
      – it can react with H-Br (forming C-H and breaking H-Br) to regenerate the allyl group and bromine radical
      – reacting with H-Br to form C-Br and the H radical is very unlikely as the H radical is more unstable than the allylic radical.

      This is why you need a source of Br2 to do an allylic bromination

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