Effect of metal ion complexation on the rate of the alkaline ester hydrolysis of 4′-(4-nitrophenoxycarbonyl)benzo-15-crown-5 in ethanol–water mixtures

Abstract

The second-order rate coefficient for the hydroxide ion-promoted hydrolysis of 4′-(4-nitrophenoxycarbonyl)benzo-15-crown-5 in 54%(w/w) ethanol–water is increased by addition ol NaCl, KCl, RbCl and CsCl. For each salt a limiting value of the rate coefficient is reached at high concentrations of salt. The limiting values are up to two times higher than the rate coefficient ir the absence of salt and decrease in the order Na⁺ > K⁺ > Rb⁺ > Cs⁺. For reaction in the presence of K⁺ the limiting value decreases as the solvent is changed from 54% to 49% and to 39%(w/w) ethanol-water. The kinetic results are compatible with the formation of a complex between the crown ester and the metal ion which is more susceptible to alkaline hydrolysis than the uncomplexed crown ester. The crown ester is unable to bind Li⁺ and the presence of LiCl results in a small reduction in the rate. For the other cations, the results have been analysed to give values for the equilibrium constant for complex formation (K) and the rate coefficient (k⁰_c) for hydrolysis of the crown ester/cation complexes. After allowance is made for electrolyte effects, it is found that the rate coefficients (corrected to infinite dilution) for hydrolysis of the crown ester/catior complexes are seven to twelve times greater than that for hydrolysis of the uncomplexed crowr ester. The trends in the values of K and k⁰_c with cation size and solvent are discussed.