The effect of the structure of the cation of an ionic liquid on the rate of reaction at a phosphorus centre

The effect of a series of bis(trifluoromethanesulfonyl)imide-based ionic liquids containing different cations on the reaction of ethanol-d(6) with diethyl chlorophosphate was investigated. The addition of each of the salts increased the rate constant, with the extent of this increase being dependent on the mole fraction of the ionic liquid in the reaction mixture and, to a lesser extent, the nature of the cation; less bulky cations with ready access to the charged centres facilitated the greatest increases in the rate constant. Temperature-dependent kinetic analyses demonstrated that the rate enhancements were due to a decrease in the activation enthalpy offsetting a decrease in the activation entropy; this basis of the rate enhancement was consistent for all of the ionic liquids used and suggests stabilisation of the transition state of the process along with associated ordering of the solvent. Copyright (c) 2016 John Wiley & Sons, Ltd.