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

TL;DR

This study investigates how solvation effects influence the hydrolysis kinetics of methylene chloride in water across a wide temperature range, combining experiments, theory, and ab initio calculations to develop a predictive rate model.

Contribution

It introduces a comprehensive approach integrating Kirkwood theory and ab initio modeling to explain and quantify solvation effects on reaction kinetics under varying conditions.

Findings

Significant hydrolysis occurs at subcritical temperatures, minimal at supercritical.

Reaction rate slows dramatically with increasing temperature due to dielectric changes.

A correction factor to Arrhenius law accurately predicts experimental data.

Abstract

The nature of the CH2Cl2 neutral/acidic hydrolysis reaction from ambient to supercritical conditions (25 C to 600 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 reaction. Experiments reveal that significant CH2Cl2 hydrolysis occurs under subcritical temperatures, while relatively little hydrolysis occurs under supercritical conditions. These trends cannot be explain 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 three orders of magnitude. These solvent effects are captured quantitatively in a correction factor to the Arrenius 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.