Kinetics of the solvolysis of trans-dichlorobis(1,2-diaminoethane)cobalt(III) ion in water and in water + propan-2-ol

Abstract

Rates of the first-order solvolysis of trans-dichlorobis(1,2-diaminoethane)cobalt(III) ions have been measured for a range of temperatures in water and in mixtures of water with propan-2-ol, a co-solvent known for its ability to induce the formation of solvent structure at low concentrations. A plot of log (rate constant) against the Y-factor is linear, but a non-linear plot is found for log (rate constant) against reciprocal of dielectric constant. Minima in the transition-state parameters observed at mole fractions of propan-2-ol where the decrease in the partial molar volume of propan-2-ol has a minimum suggest that solvent structure has a considerable influence on the rate. The application of a free-energy cycle to the process (initial state)→(transition state) in water and in the mixture shows that changes in solvation of the transition state have a dominant effect on the rate in this region of solvent composition.