Chapter 16 (a) Key - CHAPTER 16 1 Pyrrole undergoes...

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CHAPTER 16 1. Pyrrole undergoes electrophilic aromatic substitution preferentially at the 2 position as illustrated by the synthesis of 2-nitropyrrole. Write resonance structures for the intermediate carbocation formed by attack of NO 2 + at the 2 and 3 positions of pyrrole. From examination of these intermediates, offer an explanation for preferential nitration at the 2 position. N H CH 3 COOH N H NO 2 H N H NO 2 H N H NO 2 H N H NO 2 H N H NO 2 + HNO 3 1 2 3 + H 2 O When nitration occurs at the 2-position, the + charge is distributed over four atoms (including nitrogen). However, when nitration occurs at the 3-position, the + charge is distributed over two atoms (including nitrogen). Greater the delocalization of the + charge, greater is the stability of the carbocation intermediate. The more stable carbocation intermediate is more readily formed (Hammond’s postulate) and for this reason, nitration occurs at the 2- position (leads to the more stable intermediate).
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This note was uploaded on 04/07/2010 for the course BIO 3223 taught by Professor Butt during the Spring '10 term at University of South Africa.

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Chapter 16 (a) Key - CHAPTER 16 1 Pyrrole undergoes...

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