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

Abstract

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

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

“…The reaction of diethyl chlorophosphate 3 with ethanol‐ d 6 to afford the triethyl phosphate 4 , which has been studied in mixtures of molecular solvents and ionic liquids, including the salt 5 …”

“…As observed previously, it is important to recognise that there are potential competing reactions of the chloride 3 that may occur. Adventitious water may result in hydrolysis to generate diethyl hydrogen phosphate (“the diester”) whilst nucleophilic substitution at a methylene group (S N 2@C) would result in a phosphochloridate, which might further react with ethanol to also generate the diester.…”

“…The ionic liquids 6‐9 used in this work, along with the ionic liquids 5 and 10 , which have been investigated previously [15a] …”

“…Previous work in our research group has considered the reaction of diethyl chlorophosphate 3 with ethanol‐ d 6 to afford the partially deuterated triethylphosphate 4 (Scheme ), which proceeds through a bimolecular substitution mechanism, in mixtures of molecular solvents and the ionic liquids, including 1‐butyl‐3‐methylimidazolium bis (trifluoromethylsulfonyl)imide ([bmim][(CF 3 SO 2 ) 2 N], 5 ) [15a] . As the mole fraction of the salt 5 in the reaction mixture was increased, the rate constant for the process increased up to a maximum value at a mole fraction of ca .…”

“…With this in mind, the trend in rate constants described earlier for the reaction shown in Scheme in mixtures containing the salt 5 was mirrored when a series of ionic liquids containing the [(CF 3 SO 2 ) 2 N] − anion were considered [15b] . In cases where the cation had comparable steric bulk to the [bmim] + cation, the rate constants were the same as when the salt 5 was present.…”