Understanding Trends in C-H, N-H, and O-H Bond Dissociation Enthalpies

K. U. Ingold

National Research Council of Canada, Steacie Institute for Molecular Sciences, 100 Sussex Drive, Ottawa, ON K1A 0R6, Canada and    

J. S. Wright

Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Dr., Ottawa, ON K1S 5B6, Canada    

Abstract

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.

Journal of Chemical Education, 2000, 77, 1062            Full text (PDF)

  -- SiripornJu - 08 Mar 2007