(54a) Kinetics and Mechanism of N-Substituted Amide Hydrolysis in High-Temperature Water

N-methylacetamide (NMA) was selected as a model N-substituted amide to investigate the hydrolysis kinetics and mechanism of such compounds in high temperature water (HTW) with and without added acid or base catalysts. We explored temperatures between 200 and 400℃. The major products are acetic acid and methylamine, and the reaction is reversible. Batch reactor studies revealed that the hydrolysis reaction is first order in water and first order in NMA at both subcritical and supercritical conditions. From the temperature dependence of the rate constants and equilibrium constant we determined the values of the activation energy and reaction enthalpy. The hydrolysis rate is also pH dependent, and three distinct regions of pH dependence exist. At low and high pH, the conversion increased rapidly with added acid and base, respectively. At near-neutral pH, however, the rate was essentially insensitive to changes in pH. Further investigations also found that the rate constant for the hydrolysis of NMA increased with increasing water density and with the addition of salts. These results are consistent with a polar hydrolysis reaction mechanism wherein the rate constant would be increased with increases in the solvent polarity. An SN2 mechanism with water as the nucleophile appears to be the most likely candidate.