Effects of solvent and substituents on the kinetics of decomposition of phosphonium cations under the action of strong alkalis in water-methanol and water-dioxane media

The rate constants of the 3rd order reactions have been measured for the reaction between 3-bromopropyltriphenylphosphonium bromide and sodium hydroxide in water-methanol and water-dioxane media of various compositions. The reaction rate increases continuously with decreasing water content of the solvent. The dominant effect is the extent of solvation of the hydroxyl ions in methanol and dioxane media. The reaction in water-dioxane media is always faster than in methanol-water media and the difference is dependent on solvent composition, where the solvent structure and solvating power vary. The rate of decomposition of 3-bromopropyltriphenylphosphonium is larger than that of tetraphenylphosphonium ion in similar conditions, which is attributed to an inductive effect increasing the positive charge on phosphorus upon introduction of the 3-bromopropyl group. The desolvation of reactants and the solvation of transition states of the two reactions in the two solvents considered were discussed.