KINETICS,MEDIUM, AND DEUTERIUM ISOTOPE EFFECTS IN THE ALKALINE DECOMPOSITION OF QUATERNARY PHOSPHONIUM SALTS III1 Tetraphenylphosphonium Chloride in Dimethylsulphoxide-Water Mixtures

Abstract

The reaction between tetraphenylphosphonium chloride and hydroxide or deuteroxide anions was studied kinetically in a series of dimethylsulphoxide-water mixtures at several temperatures. The rate is first-order in the phosphonium cation and second-order in the hydroxide or deuteroxide anions. The reaction shows a dramatic increase in rate, up to about 10¹⁰ times, as the DMSO content is increased. The rate enhancement is attributed to a considerable drop in activation energy affected not only through an increased desolvation of reactant anions, but also through an increase in solvation of the transition state, brought about by gradual addition of DMSO. The kinetic solvent deuterium isotope effect in 60% DMSO-40% D₂O is strongly dependent on temperature. The rate constant in the latter solvent mixture is represented by k ⁱ = 11.9 e ^?12700/RT l ² mole^?2 sec^?1 as compared to k ⁱ = 19.0 e ^?22500/RT l ² mole^?2 sec^?1 in the corresponding 60% DMSO-H₂O mixture. The thermodynamic parameters of activation show strong dependence on solvent composition and are related to structural changes and solvation power of the reaction medium.