Ch_322b_13.02

Ch_322b_13.02 - Chapter 13.2 lecture note

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Allylic Bromination with N-Bromosuccinimide (NBS): A Useful Laboratory Reagent N-Bromosuccinimide is a derivative of succinimide: C O = OH C OH succinic acid O O succinic acid anhydride O O N-H succinimide O O N- Br N-bromosuccinimide (NBS) O O NBS reacts with HBr to produce Br 2 : N- Br : + H-Br NBS O O H Br + + :Br: - N O O N- H + Br-Br succinimide O O
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Allylic Bromination The bromination of the allylic position is carried out in the presence of a catalytic amount of peroxide (ROOR), or under irradiation to initiate radical production. overall CH 2 =CH- CH 3 + : h ! or ROOR CCl 4 N- Br O O CH 2 =CH- CH 2 Br + N- H O O
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A Mechanism for Allylic Bromination with NBS Initiation of the free radical chain reaction may be by any or all of the reaction sequences below: RO-OR h ! or heat 2 RO . homolysis CH 2 =CH-CH 3 + RO . CH 2 =CH-CH 2 . + RO-H chain carrier or Br-Br h ! or heat homolysis 2 Br . chain carrier or N- Br h ! or heat homolysis N : . + Br . CH 2 =CH-CH 3 + N . CH 2 =CH-CH 2 . + chain carrier N-H chain carrier O O O O O O O O
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Propagating Steps (1) CH 2 =CH-CH 3 + Br . CH
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Ch_322b_13.02 - Chapter 13.2 lecture note

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