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Information • Textbooks • Media • Resources
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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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Hugh J. Anderson
Department of Chemistry, Memorial University, St. John's, NF A1B 3X7, Canada
Ludwig Bauer
College of Pharmacy, M/C 781, University of Illinois at Chicago, 833 S. Wood Street, Chicago IL 60612-7231
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August 1999
Vol. 76 No. 8
p. 1151
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| Abstract |
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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 6p-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 6p- 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.
Pyridine is classified correctly as a weak base, while pyrrole in aqueous solutions is classified essentially neutral. In reality, pyrrole is a weak acid, neutralized by strong bases to generate a stable aromatic and highly reactive nucleophilic anion. Most authors fail to stress the acidic property of pyrrole and some organic texts need revision to eliminate some misconceptions and omissions.
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| More Information |
 Citation |
Anderson, Hugh J.; Bauer, Ludwig. J. Chem. Educ. 1999 76 1151.
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Keywords |
Chemical Education Research; Organic Chemistry; AcidÐBase Chemistry; Textbooks
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History |
Created:
Last Updated: |
July 8, 1999
June 23, 2005
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| Home > JCE Print > Journal of Chemical Education > Issues >
1999
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