Effect of metal-ion complexation on the rate of the alkaline ester hydrolysis and the carbon acid dissociation of crown malonates in ethanol–water mixtures

Abstract

The initial rates of the alkaline hydrolysis of 19-crown-6 malonate (1) and 16-crown-5 malonate (2) in ethanol–water mixtures are increased to limiting values by the addition of NaCl and KCl. For 1 in 76%(w/w) ethanol–water the addition of 0.015 mol dm^–3 KCl produces a 65-fold increase in the rate compared with that in the absence of metal ions. The results are compatible with the formation of metal ion–crown malonate complexes which undergo hydrolysis more rapidly than the uncomplexed ligands. Values of the equilibrium constants (K) for complex formation and values of the rate coefficients for hydrolysis of the complexes at hypothetical zero ionic strength (k°_c) are derived. For the K⁺–19-crown-6 malonate complex the values of K and k°_c/k_f, where k_f, is the rate coefficient for hydrolysis of the uncomplexed ligand, increase as the relative permittivity (ε_r) of the solvent mixture decreases. The magnitude of the k°_c/k_f values indicate that the relative permittivity of the space between the bound cation and the reaction centre is about twofold lower than the value of ε_r for the bulk solvent. Spectrophotometric results for the formation of carbanions from 1 and 2 permit qualitative conclusions to be reached about the effect of cation binding on the dissociation of the crown malonates as carbon acids.