The transition-state acidity and carbonyl stretching frequency in esterhydrolysis. A substituent-effect approach

Abstract

The acidity of the transition-state complex in the acid-catalysed hydrolysis of esters increases by the effect of electron-withdrawing groups in either the acyl or alkyl moieties. From the pKa of 13 transition-state complexes, the vibrational frequency of the carbonyl oscillator in the transition state was found equal to 1300 ± 100 cm-1, the same as in the cases of saponification of methyl formate and the methoxide-catalysed methanolysis of p-bromophenyl benzoate. This suggests that the transition-state complexes of both lyonium and lyate catalysed reactions share similar structural features.