Comments on the Treatment of Aromaticity and Acid-Base Character of Pyridine and Pyrrole in Contemporary Organic Chemistry Textbooks

Presentations of aromaticity and acid-base character of pyridine and pyrrole in 18 contemporary organic chemistry textbooks were surveyed. Superficially, each of these two monoaza aromatic heterocycles retains an electron pair on its sp2-hybridized ring nitrogen, ostensibly available for neutralizat...

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Bibliographic Details
Published in:Journal of chemical education 1999-08, Vol.76 (8), p.1151
Main Authors: Bauer, Ludwig, Anderson, Hugh J
Format: Article
Language:English
Subjects:
Acids
Aromaticity
Chemistry
Organic chemistry
Personality
Science education
Textbooks
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Summary:Presentations of aromaticity and acid-base character of pyridine and pyrrole in 18 contemporary organic chemistry textbooks were surveyed. Superficially, each of these two monoaza aromatic heterocycles retains an electron pair on its sp2-hybridized ring nitrogen, ostensibly available for neutralization with a proton to form a salt. The two nonbonded electrons in pyridine, in an sp2-hybridized orbital outside the ring, are not interacting with the 6π-electron aromatic sextet and thus are fully available for neutralization to form an N+-H pyridinium cation. However, in pyrrole, the nonbonded electron pair on nitrogen is part of the aromatic 6π- electron sextet and is not available for NH salt formation, since this would generate an aliphatic (highly reactive) diene imminium cation, which would destroy pyrrole's aromaticity. However, electrophilic attack of a proton in an irreversible manner attacks C-2 of pyrrole to form a resonance-stabilized cationic intermediate capable of further transformations.
ISSN:0021-9584
1938-1328
DOI:10.1021/ed076p1151