(13)C NMR chemical shifts and nu(C[double bond]O) frequencies have been measured for several series of phenyl- or acyl-substituted phenyl acetates and for acyl-substituted methyl acetates to investigate the substituent-induced changes in the electrophilic character of the carbonyl carbon. Charge density, bond order, and energy calculations have also been performed. The spectroscopic and charge density results indicate that opposite to the conventional thinking, electron-withdrawing substituents do not increase the electrophilicity of the carbonyl carbon but instead decrease it. On the other hand, reaction energies of the isodesmic reactions designed show that electron-withdrawing substituents destabilize the carbonyl derivatives investigated. So, a significant ground-state destabilization of carboxylic acid esters, and carbonyl compounds in general, due to the decreased resonance stabilization, is proposed as a novel concept to explain both the increase in their reactivity and the changes in the chemical shifts and carbonyl frequencies induced by electron-withdrawing substituents.

Rate coefficients of nucleophilic acyl substitutions, carboxylate carbon 13 C NMR chemical shift values and ν(C᎐ ᎐ O) frequencies of several series of aryl and acyl substituted aryl acetates or alkyl benzoates have been investigated. An increasing electron-withdrawal by the acyl or aryl substituents results in higher reaction rates, upfield 13 C NMR chemical shifts and higher frequencies of the C᎐ ᎐ O stretching. Good correlations are observed for the log k versus δ C (C᎐ ᎐ O) plots. The increase of the reaction rate with increased electron density at the C᎐ ᎐ O carbon (as proved by 13 C NMR shifts) contradicts the previous concept of increased electrophilicity of the carbonyl carbon by electronwithdrawing substituents. The rate increase is now attributed to the decrease of the ester ground state resonance stabilization caused by electron-withdrawing substituents. The use of log k versus δ C (C᎐ ᎐ O) correlations is presented as a practical method to evaluate rate coefficients especially for compounds for which Hammett type correlations cannot be used.

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Carbonyl carbon (13)C NMR chemical shifts delta(C)(C[double bond]O) measured in this work for a wide set of substituted phenyl benzoates p-Y-C(6)H(4)CO(2)C(6)H(4)-p-X (X = NO(2), CN, Cl, Br, H, Me, or MeO; Y = NO(2), Cl, H, Me, MeO, or NMe(2) ) have been used as a tool to study substituent effects on the carbonyl unit. The goal of the work was to study the cross-interaction between X and Y in that respect. Both the phenyl substituents X and the benzoyl substituents Y have a reverse effect on delta(C)(C[double bond]O). Electron-withdrawing substituents cause shielding while electron-donating ones have an opposite influence, with both inductive and resonance effects being significant. The presence of cross-interaction between X and Y could be clearly verified. Electronic effects of the remote aromatic ring substituents systematically modify the sensitivity of the C[double bond]O group to the electronic effects of the phenyl or benzoyl ring substituents. Electron-withdrawing substituents in one ring decrease the sensitivity of delta(C)(C[double bond]O) to the substitution of another ring, while electron-donating substituents inversely affect the sensitivity. It is suggested that the results can be explained by substituent-sensitive balance of the contributions of different resonance structures (electron delocalization, Scheme 1).