Effects of solvent and ionic medium on the kinetics of axial ligand substitution in vitamin B12. Part VII.The reaction between aquanitrocobaloxime and thiourea in dioxane-water mixtures

Rate constants for the reaction of aquanitrocobaloxime with thiourea were measured as a function of pH, solvent composition, pressure and temperature, in dioxane-water mixtures. With the aid of solubility measurements a complete quantitative analysis of solvent effects on the initial state and transition state transfer parameters could be made. It was found that the activation enthalpy and entropy vary strongly with solvent composition. This is in contrast to the variations found for vitamin B12, for which this cobaloxime is a model compound. The rate constants increase strongly after 50 vol.% dioxane in the dioxane-water mixtures, another difference with vitamin B12. The volumes of activation are small and positive, in accordance with a dissociative mode of activation.