Kinetic medium effects. IV. Hydrolysis of trimethyl phosphate in mixed solvents

Abstract

The rate of hydrolysis of trimethyl phosphate (tmp) has been measured at 90.5º as a function of solvent composition in water-dimethyl sulphoxide and water-ethylene glycol mixtures. The rate constant of the "spontaneous" hydrolysis decreases with decreasing water content; an acid-catalysed reaction, whose rate constant increases with decreasing water content, becomes dominant at low water concentrations. Solvent isotope effects in D₂O have been measured for both the spontaneous and acid-catalysed reactions, and are close to unity in each case. Salt effects have also been studied In aqueous sodium nitrate and perchlorate solutions, and are found to be small. Reaction mechanisms are discussed in the light of the results obtained.