Correlation analysis of carbonyl carbon 13C NMR chemical shifts, IR absorption frequencies and rate coefficients of nucleophilic acyl substitutions. A novel explanation for the substituent dependence of reactivity

Abstract

Rate coefficients of nucleophilic acyl substitutions, carboxylate carbon ¹³C NMR chemical shift values and ν(CO) 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 ¹³C NMR chemical shifts and higher frequencies of the CO stretching. Good correlations are observed for the log kversus δ_C(CO) plots. The increase of the reaction rate with increased electron density at the CO carbon (as proved by ¹³C NMR shifts) contradicts the previous concept of increased electrophilicity of the carbonyl carbon by electron-withdrawing 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 kversus δ_C(CO) correlations is presented as a practical method to evaluate rate coefficients especially for compounds for which Hammett type correlations cannot be used.