Pure and Applied Chemistry

Pure Appl. Chem., 2013, Vol. 85, No. 5, pp. 921-940

http://dx.doi.org/10.1351/PAC-CON-13-01-03

Published online 2013-04-29

Hyperconjugation in hydrocarbons: Not just a “mild sort of conjugation”*

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• 40. Although various ways of quantifying the strength of a chemical bond are available, “bond energies (BEs),” “bond dissociation energies,” and theoretical “in situ bond strengths” (cf. ref. [77]), are evaluated by different procedures and have very different chemical meanings. Bond energies are derived from atomization energies (AEs) assuming that these can be divided into contributions from all the individual two-center, two-electron chemical bonds. BEs can be obtained directly from experimental data only for molecules with a single bond type [e.g., diatomic molecules, methane (AE divided by 4), ammonia (AE divided by 3)]. Since ethane has two types of bonds (six equivalent CHs and one CC), additional assumptions or data are needed to derive the BEs. For example, if one assumes that the CH BEs of methane and of ethane are equal (as shown in ref. [42]), then the C–C BE of ethane is 79.0 kcal/mol (using the current CCCBDB-recommended experimental data, 298 K). Based on the same assumption, that the CH BEs of methane and of ethylene are equal, the C=C BE of the latter is 140.9 kcal/mol. It is commonly assumed (incorrectly, since the bond lengths are quite different, see ref. [44]) that the σ-CC component BE of ethylene equals the BE of ethane. On this basis, the π-bond energy of the ethylene π-CC component is 140.9 – 79.0 = 61.9 kcal/mol, a value that matches the ethylene rotational barrier reasonably well. If one assumes that, due to hybridization differences, the C_sp2–H BE of ethylene is 2 kcal/mol larger than the C_sp3–H BE of methane and ethane (see ref. [42]), then the C=C BE of ethylene is 132.9 kcal/mol, and the π-CC BE is even weaker (132.9 – 79.0 = 53.9 kcal/mol). Bond dissociation energies are the standard enthalpy change of homolytic bond cleavage, but this includes both the intrinsic BE of the cleaved bond and the reorganization energy of the dissociated fragments. Following the valence bond definition for a chemical bond, computed “in situ bond strengths” provide a “direct measure of the stabilization associated to the exchange of the two electrons in a covalent valence bond structure” “complemented by the resonance energy stabilization owing to the mixing of ionic structures” [77]. The three procedures described above differ drastically; e.g., the σ-C–C bond energy, bond dissociation energy, and in situ σ-bond strength, of ethane are: 79.0 kcal/mol (see above; or 86.6 kcal/mol in ref. [42]), 96.1 kcal/mol [77], and 138.5 kcal/mol [77], respectively. This discussion will be amplified in a separate paper.
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