Solvent Effects on the Kinetics and Mechanism of the Acid-Catalysed Hydrolysis of Diterf.-butylsuccinate in Dioxane-Water Mixtures

Abstract

The acid-catalysed hydrolysis of ditert.-butylsuccinate in dioxane-water mixtures proceeds via consecutive first-order reactions. The rate constants kI and kII corresponding to the two steps decrease with increasing dioxane content of the medium. After reaching a minimum at 90% (w/w) dioxane, both rate constants increase again with further addition of dioxane. The kinetic ratio kI/kII is smaller than the statistical value 2.0, and is markedly affected both by solvent composition and temperature. The maximum concentration of the intermediate half ester decreases with increasing dioxane content. The observed activation energies EI and EII of the two steps of the reaction are largely dependent on temperature as well as solvent composition indicating a mixed type of bond fission represented by the AAl 1 and AAC 2 mechanisms, whose relative contributions in the overall rate constants kI and kIIcould be calculated on the basis of the number of water molecules incorporated in the corresponding transition states. The effect of bulk dielectric constant on the reaction rate was investigated in the light of the available electrostatic theories and showed the reaction to be an ion-molecular dipole type of interaction. The activation thermodynamic parameters were computed and discussed as criteria of solvent effect and mechanism.