Kinetics, medium, and deuterium isotope effects in the alkaline decomposition of quaternary phosphonium salts. IV. Tetraphenylphosphonium chloride in tetrahydrofuran-water mixtures

Abstract

The third-order rate constants of the reaction between tetraphenylphosphonium chloride and sodium hydroxide in binary tetrahydrofuran—water systems have been determined as a function of temperature. Increasing the THF content of the reaction medium up to 70% (v/v) results in a ca. 106-fold rate acceleration at 25 °C. This is explained as due to increased desolvation of reactants as well as an opposite effect on the transition-state solvation in THF-rich media. The rate in deuterium oxide is higher than in water and the kinetic isotope factor k′DO−/k′110− depends strongly on temperature. The inverse proportionality of rate to the water concentration of medium showed that a large number of water molecules (about 13) is to be removed from the solvation sheaths of reactants before formation of the activated complex. The thermodynamic activation parameters have been derived and expressed as functions of the composition of the medium. It is found that ΔH≠ decreases by about 17 kcal/mol over the range 0–70% THF, while ΔS≠ decreases by about 29 cal mol−1 deg−1 over the same range. As a result, ΔG≠ shows a net decrease of 8 kcal/mol indicating partial compensation between ΔH≠ and TΔS≠.