Solvent effect on activation parameters for intramolecular general base-catalysed hydrolyses of salicylate esters and hydroxide ion-catalysed hydrolysis of methyl p-hydroxybenzoate

Abstract

The effects of varying concentrations of methyl alcohol, dimethyl sulphoxide, t-butyl alcohol, and acetonitrile on activation parameters for intramolecular general base-catalysed aqueous cleavages of methyl salicylate and alkaline hydrolysis of methyl p-hydroxybenzoate have been studied. The activation parameters for intramolecular general base-catalysed cleavage of phenyl salicylate was studied under varying concentrations of t-butyl alcohol, dimethyl sulphoxide, and acetonitrile. A significant enthalpy–entropy compensation effect is observed in the hydrolyses of these esters with change of t-butyl alcohol content into the reaction medium. The variation of acetonitrile concentration causes an enthalpy–entropy compensation effect only in the alkaline hydrolysis of methyl p-hydroxybenzoate. A significantly larger negative value of ΔS*, observed in the hydrolysis of methyl p-hydroxybenzoate at 90% H₂O (v/v), compared with those for hydrolyses of salicylate esters obtained under an essentially similar solvent composition, is attributed to the difference in mechanisms for hydrolysis of these esters.