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 °C. The binary solvent composition covers [H 2 O] from 0.02 to 51.39 M, corresponding to the water mole fraction, χ w, from 0.100 to 0.971. The dependence of log ( k obs ), the observed rate constant, on χ w is sigmoidal and is similar to the dependence of the solvent polarity scale E T (30) on χ w for the same solvent mixture. As a function of decreasing χ w, the Gibbs free energy of activation gradually increases, but Δ H ⧧ and Δ S ⧧ show a complex, quasi-mirror image dependence on χ 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 χ 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 χ w and E T (30) versus χ 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.