The influence of cation binding on the kinetics of the hydrolysis of crown ether acetals

Abstract

The rate of hydrogen ion-catalysed hydrolysis of crown ether acetals in 60 : 40 (v/v) dioxan–water is found to be strongly decreased by the addition of alkali and alkaline earth metal chlorides having cations of appropriate size to be complexed by the substrate ring. The compounds studied are the monoacetals CH₃[graphic omitted] with x= 1–8. The dependence of the initial rate of formation of acetaldehyde on metal-ion concentration is expressed in terms of (i) the equilibrium constant for complex formation, (ii) the influence of the bound cation on the rate constant, and (iii) an electrolyte effect. A curve-fitting procedure is used to derive the parameters governing the first two of these effects. The equilibrium constants are large and cannot be evaluated with any precision, but a fair estimate of the influence of the guest cation on the rate can be obtained. This effect is explicable by the electrostatic repulsion between the cationic charges of the metal ion and the proton added to the acetal in the first step of the hydrolysis. The size of the effect requires the values of the effective relative permittivity of the space between the charges to be close to that of the bulk solvent.