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Chemistry LibreTexts

18.13 Benzylic Halogenation

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  • The benzylic C-H bonds weaker than most sp3 hybridized C-H. This is because the radical formed from homolysis is resonance stabilized.

    Resonance stabilization of the benzylic radical

    Because of the weak C-H bonds, benzylic hydrogens can form benzylic halides under radical conditions.

    NBS as a Bromine Source

    NBS (N-bromosuccinimide) is the most commonly used reagent to produce low concentrations of bromine. When suspended in tetrachloride (CCl4), NBS reacts with trace amounts of HBr to produce a low enough concentration of bromine to facilitate the allylic bromination reaction.

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    Allylic Bromination Mechanism

    Step 1: Initiation

    Once the pre-initiation step involving NBS produces small quantities of Br2, the bromine molecules are homolytically cleaved by light to produce bromine radicals.


    Step 2 and 3: Propagation

    Step 4: Termination


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