Effects of Ionic Liquids on the Nucleofugality of Chloride

Abstract

The nucleofugality of chloride has been measured in solvent mixtures containing ionic liquids for the first time, allowing reactivity in these solvents to be put in context with molecular solvents. Using well-described electrofuges, solvolysis rate constants were determined in mixtures containing different proportions of ethanol and the ionic liquid 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)imide; the different solvent effects observed as the mixture changed could be explained using interactions of the ionic liquid with species along the reaction coordinate, determined using temperature dependent kinetic studies. The solvolysis data allowed determination of the nucleofugality of chloride in these mixtures, which varied with the proportion of salt in the reaction mixture, demonstrating quantitatively the importance of the amount of ionic liquid in the reaction mixture in determining reaction outcome. Nucleofugality data for chloride were determined in seven further ionic liquids, with the reactivity shown to vary over more than an order of magnitude. This outcome illustrates that the components of the ionic liquid are critical in determining reaction outcome. Overall, this work quantitatively extends the understanding of solvent effects in ionic liquids and demonstrates the potential for such information to be used to rationally select an ionic liquid to control reaction outcome.

The effects of ionic liquids on the ethanolysis of a chloroacenaphthene. Evaluation of the effectiveness of nucleofugality data to predict reaction outcome

“…1) in ethanol at 40 °C. The ionic liquid 3 was chosen as it is readily prepared and puried, 39 and has been widely utilised in mechanistic studies; 21,38,[40][41][42] of particular note is its previous use to evaluate solvent effects on reactions that also proceed through the S N 1 mechanism. 13,18,19,34 The temperature was chosen such that the reactions could be readily followed based on previously observed solvent effects of ionic liquids.…”

“…13,18,19,34 The temperature was chosen such that the reactions could be readily followed based on previously observed solvent effects of ionic liquids. 11 While this value is different to that used in nucleofugality studies, 39 they would still be expected to provide useful quantitation data (vide infra). The reaction was followed in situ using 1 H NMR spectroscopy and the kinetic data obtained are summarised as a plot showing the mole fraction dependence of the rate constant (Fig.…”

“…This dependence of reaction outcome on solvent composition, with a rate constant increase followed by a decrease, is similar to what has been observed previously for other processes that proceed through an S N 1 mechanism. 13,18,19,22,30,34,39 Within this framework, there is signicant variation in both the extent of rate constant increase (from 1.5 (ref. )) and the nal rate constant relative to the neat molecular solvent (from 1000% 13 of the rate in molecular solvent to 1% 30 ).…”

“…The data presented here are consistent with what has been seen previously for chloride nucleofuges with systems that have a signicant degree of charge stabilisation of the carbocation intermediate. 22,39 Temperature dependent kinetic studies…”

“…We have recently started to determine quantitative reactivity parameters in ionic liquids and their mixtures, particularly the nucleofugality of chloride. 39 Nucleofugality measures the ability of a leaving group to depart with a bonding electron pair and can be quantied using the relationship:…”