Kinetics of the pH-independent hydrolysis of bis(2,4-dinitrophenyl) carbonate in acetonitrile-water mixtures: Effects of the structure of the solvent

The pH-independent hydrolysis of bis(2,4-dinitrophenyl) carbonate, DNPC, in aqueous acetonitrile was studied spectrophotometrically from 20 to 45 degrees C. The binary solvent composition covers [H2O] from 0.02 to 51.39 M, corresponding to the water mole fraction, chi(w), from 0.100 to 0.971. The dependence of log (k(obs)), the observed rate constant, on chi(w) is sigmoidal and is similar to the dependence of the solvent polarity scale E-T(30) on chi(w) for the same solvent mixture. As a function of decreasing chi(w), the Gibbs free energy of activation gradually increases, but Delta H-double dagger and Delta S-double dagger show a complex, quasi-mirror image dependence on chi(w). Plots of log (k(obs)) versus log [water] do not allow calculation of a single kinetic order with respect to water over the entire range of [water]. The structure of the transition state was probed by a proton inventory study carried out at chi(w) = 0.453, 0.783, and 0.871, respectively. Plots of observed rate constants versus the atom fraction of deuterium in the solvent curve downward, and the results were fitted to a transition-state model that contains DNPC and two water molecules. Thus, the sigmoidal dependence of log (k(obs)) on log [water] is not due to an increase in the number of water molecules in the transition state as a function of increasing [water]. The similarity of plots of log (k(obs)) versus chi(w) and E-T(30) versus chi(w) suggest similar solute-solvent interaction mechanisms, namely H-bonding and dipolar interactions. Kinetic results are discussed in terms of effects of the structure of acetonitrile-water mixtures on the solvation of reactant and transition states.