Relationship Investigation between C(sp²)-X and C(sp³)-X Bond Energies Based on Substituted Benzene and Methane

The C-X bonds of organic compounds between group X and a saturated or unsaturated carbon atom differ in bond energy. To identify the causes of variation is of great significance in terms of bond nature understanding and bond energy estimation. In this paper, the electronegativity χ[X] of group X was calculated by the "valence electron equalized electronegativity" method. Then, χ[X] and the electronic effect constant of the substituent were taken as variables to establish equations for quantitative correlation between C(sp³)-X and C(sp²)-X for the calculation of C-X bond energies. The aim is make comparison between substituted methane, Me-X, and substituted benzene, Ph-X, as well as that between Me-X and substituted ethylene, C₂H₃-X. We conducted calculation over 40 compounds that contain different X groups, and the results reveal that the C(sp³)-X and C(sp²)-X bond energies are under the influence of a number of factors. In addition to the covalent properties of C and X atoms and χ[X], the bond energies of C(sp²)-X (i.e., D[C(sp²)-X]) are under the influence of the field/inductive effect (σ_F[X]) and conjugated effect (σ_R[X]) of group X, with the former causing a decrease while the latter an increase of D[C(sp²)-X]. Using the acquired quantitative correlation equations and on the basis of a relatively rich set of measured D[Me-X] data, we estimated D[Ph-X] of Ph-X and D[C₂H₃-X] of C₂H₃-X, and the estimation accuracy is within experimental uncertainty. Employing the above method, the D[C(sp²)-X] of 33 substituted benzenes, 53 substituted ethenes, and 82 α-substituted naphthalenes was estimated with satisfactory outcomes.