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Unformatted text preview: 19- 13 19- 14 (a) � : NH� � _ jlC
-0 0 �-o:OoCH3 j;�- o�H �5Hl c �' GOOD
Cl I � 00 _ _ __ .. -Cl � N N _N N N N Br GOOD stabilized by induction from nitrogen � NH2 This is a benzyne-type mechanism. (For simplicity above, two steps of benzyne generation are shown as one step: first, a proton is abstracted by amide anion, fol lowed by loss of bromide.) Amide ion is a strong enough base to remove a proton from 3-bromopyridine as it does from a halobenzene. Once a benzyne is generated (two possibilities), the amide ion reacts quickly, forming a mixture of products. Why does the 3-bromo follow this extreme mechanism while the 2-bromo reacts smoothly by the addition e limination mechanism? Stability of the intermediate! Negative charge on the electronegative nitrogen makes for a more stable intermediate in the 2-bromo substitution. No such stabilization is possible in the 3bromo case.
N NH -� : ...
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- Spring '10