Brønsted acids in ionic liquids: how acidity depends on the liquid structure

Abstract

Gutmann Acceptor Number (AN) values have been determined for Brønsted acid–ionic liquid mixtures, over a wide compositional range. Four systems of general formula [C₂mim][A]–HA (A⁻ = bistriflamide, [NTf₂]⁻; triflate, [OTf]⁻; mesylate, [OMs]⁻; or acetate, [OAc]⁻, [C₂mim]⁺ = 1-ethyl-3-methylimidazolium cation) were studied. A library of Brønsted acidic systems of varying acidity was constructed and the AN parameter was found to be a convenient approach for quantifying their acidity. HOAc, HOMs and HOTf, when dissolved in ionic liquids, were found to associate with the respective anions to form hydrogen-bonded anionic clusters, [A(HA)_x]⁻. In contrast, HNTf₂ was solubilised as a discrete, undissociated molecule. AN values were sensitive to the presence of anionic clusters; acidity could be buffered to a particular AN by binding the solubilised acid in the anionic cluster form. Overall, a simple way to manipulate and quantify the Brønsted acidity of acid–ionic liquid mixtures was demonstrated, and measured AN values were related to liquid speciation.