The influence of cation binding on the kinetics of the hydrolysis of a crown ether-like cyclic ester

Abstract

The initial rate of the reaction in 60 : 40 (v/v) dioxan–water between hydroxide ion and the cyclic diester (1,5,8,11,14,17-hexaoxanonadecane-2,4-dione formed from malonic acid and pentaethylene glycol (3,6,9,12-tetraoxatetradecane-1,14-diol) to give the acyclic half ester is found to be strongly increased by the addition of sodium chloride, potassium chloride, rubidium chloride, and caesium chloride in low concentration. The effect is analysed in terms of complex formation between substrate and metal ion, the complexed form having a higher reactivity. Approximate values of the equilibrium constant and rate constant involved are derived by a curve-fitting procedure. The ratio of the initial rate constants for reaction of complexed and uncomplexed ester is in accordance with the electrostatic attraction between the bound metal ion and the negative charge introduced by the attack of the hydroxide ion, provided the effective relative permittivity of the space between the two charges is taken to have a value close to the relative permittivity of the bulk solvent.