CH Acidity of Di- and Trisubstituted Methanes

Abstract

It is shown that the CH acidity of di- and trisubstituted methanes can be studied using the ap- proaches consisting in singling out the contributions of physically significant factors from the overall acidification mechanisms in the gas phase and in solution. This procedure implies formal decomposition of the calculated gas-phase deprotonation energy ΔEdeprot into the following terms: electrostatic energy of proton detachment from the “acid” CH bond, with the state of the remainder of the molecule absolutely unperturbed (ΔE1); electronic relaxation energy of the resulting molecular residue and formation of a “virtual” carbanion therefrom (ΔE2); the ΔEdeprot component due to displacement of the atomic nuclei on changing from the “virtual” to real carbanion ΔE3. Relationships between the energy components ΔE1, ΔE2, ΔE3, and the commonly used characteristics of the molecular structure were investigated. The parameter ΔE1 is selectively sensitive to the inductive effect of the substituent. Imperfect correlation between ΔE1 and the sum of the σI constants can be due to the fact that the contributions to the σI constants from the effective charge on the hydrogen atom of the CH bond being deprotonated and from the polarizabilities of the substituents are not taken into account. In contrast to monosubstituted methanes, in di- and trisubstituted methanes there is no correlation between the ΔE2 component and the 1J(13CH) constants. The linear dependence linking the sums of the components ΔE1 + ΔE2 and the pyramidal angle in the carbanions is responsible for the relaxation nature of the effects associated with the ΔE2 + ΔE3 sum. Comparison of the data obtained with the calculated patterns of redistribution of the effective charges on atoms accompanying conversion of CH acids to carbanions enabled elucidation of the relative contribution of each of the components, ΔE2 and ΔE3, to the deprotonation energy of selected groups of substituted methanes. The previously developed technique of separating the energy of protolytic equilibrium in the gas from that of solvation processes in solution enabled assessment of the contributions from electrostatic solvation to pK a in DMSO. The same technique of singling out the solvation component due to intermolecular hydrogen bonds from pKa(H2O) was used in studying the acid-base equilibria for substituted methanes in aqueous solution. It was shown that the solvation effects manifested in the liquid-phase CH acidity can be modeled by the effects revealed for analogous hydrogen bonds of ion-molecule complexes in the gas phase. The relationships between the strength of hydrogen bonds and the CH-acidic properties of compounds in the gas phase and liquid water are similar.