Solvation effects on kinetics of methylene chloride reactions in sub- and supercritical water: Theory, experiment, and ab initio calculations

The nature of the CH2Cl2 neutral/acidic hydrolysis reaction from ambient to supercritical conditions (25 to 600 degrees C at 246 bar) is explored. Of primary interest is the effect of the changing dielectric behavior of the water solvent over this temperature range on this hydrolysis reaction. Experiments reveal that significant CH2Cl2 hydrolysis occurs under subcritical temperatures, whereas relatively little hydrolysis occurs under supercritical conditions. These trends cannot be explained by simple Arrhenius behavior. A combination of Kirkwood theory and ab initio modeling provides a means of successfully accounting for this behavior both qualitatively and quantitatively. The results show that increases in the activation energy and a changing reaction profile with a decreasing dielectric constant provide a mechanism for a slowing of the reaction at higher temperatures by as much as 3 orders of magnitude. These solvent effects are captured quantitatively in a correction factor to the Arrhenius form of the rate constant, which is incorporated into a global rate expression proposed for CH2Cl2 hydrolysis that provides good predictions of the experimental data.