Acid-base equilibria in ionic liquids

Ionic liquids (ILs) are low temperature molten organic salts, with melting points lying often below room temperature. Due to their peculiar properties, such as low volatility, low flammability and high stability, they have been widely used in the last two decades as alternative reaction media to conventional organic solvents for many organic reactions. Aliphatic amines and pyridines have been used as nucleophiles or bases in organic reactions performed in ILs. In some cases, rate constants are higher than those expected on the basis of the pKa values in water, suggesting a higher nucleophilicity/basicity of amines in ILs than in conventional organic solvents. It is well known that the acid-base strength depends on the solvent and it is reasonable to expect that acids and bases can differently interact with a molecular solvent and an ionic solvent. Hence, particular caution is necessary if pKa values determined in water are used to rationalize quantitative results obtained in ILs. In a previous work we have determined the equilibrium constants for ion pair formation, Kip, of some pyridines with trifluoroacetic acid in different ionic liquids. It turned out that the basicity order is the same in ionic liquids and in water and pyridine basicity is less sensitive to the substituent effect in ionic liquids than in water. In the present work we have determined the ion-pair basicity in ILs of various substituted anilines and other bases as a further extension of the earlier established basicity scale for pyridines. The ion-pair basicity has been estimated quantitatively from the aptitude of the base to take a proton from trifluoroacetic acid. The values of Kip have been measured in ILs, obtained from the combination of different cations and anions, by spectrophotometric titration, adding increasing amounts of TFA to a solution of the base. The effects of the different ILs cation/anion compositions on the ion-pair basicities have been discussed. To compare more rigorously the basicities in ILs and in water, the experimental Kip values have been corrected for ion-pairing using the Fuoss equation to obtain relative ionic basicities, Ka