Ch_322b_21.07

Ch_322b_21.07 - Chapter 21.7 lecture note

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The Elimination-Addition or Benzyne Mechanism Nucleophilic aromatic substitution occurs in the absence of "activating substituents" in the presence of very strong bases . Br + K + - NH 2 -33 o C liquid NH 3 NH 2 + KBr Positional Label Experiments In experiments where the positions of the ring carbons can be followed, suprising results are obtained. Br + K + - NH 2 -33 o C liquid NH 3 NH 2 CH 3 CH 3 NH 2 CH 3 + Br + K + - NH 2 -33 o C liquid NH 3 is 14 C radiolabel NH 2 NH 2 + 50% 50%
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The Benzyne Mechanism Br + K + - NH 2 -33 o C liquid NH 3 Br H + - : NH 2 . . elimination benzyne + NH 3 + Br - (a reactive intermediate) In the presence of very strong bases, the elimination-addition or benzyne mechanism operates. In this mechanism, the strong base promotes an elimination reaction producing a highly reactive intermediate called a benzyne (" benz ene" + " yne'' ). A nucleophile (usually the strong base) then adds to the benzyne intermediate producing a phenyl carbanion. The above results are inconsistent with the previously described
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_21.07 - Chapter 21.7 lecture note

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