Understanding Trends in C-H, N-H, and O-H Bond Dissociation Enthalpies
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J. S. Wright [1] ; K. U. Ingold [2]
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[1] Carleton University
Carleton University
Canadá
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[2] National Research Council Canada
National Research Council Canada
Canadá
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- Localización: Journal of chemical education, ISSN 0021-9584, Vol. 77, Nº 8 (August), 2000, págs. 1062-1062
- Idioma: inglés
- Texto completo no disponible (Saber más ...)
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Resumen
Theory and experiment are now in good agreement for bond dissociation enthalpies (BDE's) and can be used as checks on each other to identify anomalous data. Both theory and experiment agree that XO-H, XNH-H, and XCH2-H BDE's decrease monotonically by ca. 60 kJ/mol along the series for X = H, but by only ca. 13 kJ/mol for X = CH3. More surprising is the fact that for X = C6H5 the O-H bond is the weakest and the N-H bond is the strongest (by 10-15 kJ/mol). By contrast, BDE's are essentially identical for X = CF3 and X = H. These interesting trends are discussed in terms of a number of concepts useful in advanced organic chemistry courses, including electronegativity, bond length, orbital overlap, unpaired electron delocalization by conjugation and hyperconjugation, and radical stabilization energy. The discussion is supported by EPR data for unpaired electron spin densities and the most recent thermochemical data for BDE's.
