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

Gutmann Acceptor Number (AN) values have been determined for Broensted acid-ionic liquid mixtures, over a wide compositional range. Four systems of general formula [C sub(2)mim][A]-HA (A super(-) = bistriflamide, [NTf sub(2)] super(-); triflate, [OTf] super(-); mesylate, [OMs] super(-); or acetate, [OAc] super(-), [C sub(2)mim] super(+) = 1-ethyl-3-methylimidazolium cation) were studied. A library of Broensted 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) sub(x)] super(-). In contrast, HNTf sub(2) 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 Broensted acidity of acid-ionic liquid mixtures was demonstrated, and measured AN values were related to liquid speciation.