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

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Ludwig Bauer

College of Pharmacy, M/C 781, University of Illinois at Chicago, 833 S. Wood Street, Chicago IL 60612-7231

Hugh J. Anderson

Department of Chemistry, Memorial University, St. John''s, NF A1B 3X7, Canada

J. Chem. Educ., 1999, 76 (8), p 1151

DOI: 10.1021/ed076p1151

Publication Date (Web): August 1, 1999

Abstract

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.

Keywords (Audience):

Second-Year Undergraduate

Keywords (Domain):

Organic Chemistry

Keywords (Pedagogy):

Textbooks / Reference Books

Keywords (Subject):

Acids / Bases

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