Hammett Acidity Scale in Ionic Liquids : An Indication of Their Weak Dissociating Character

Ionic liquids are salts with the particularity to exhibit melting points near room temperature (below 100°C, by convention) with no vapour tension. For this last reason, ionic liquids are sometimes called “green solvents”. In addition, their exclusive materials and solvent properties has led to an amazing increase of interest from both academic and industrial community, confirmed by the explosion of the number of published papers in the last decade.

The many combinations of organic and inorganic cations and anions allow an infinity of new ionic solvents then permitting the selection of the desired properties for a given application. Nevertheless, it is impossible to investigate all these combinations and the unusual complexity of these new solvents gives rise to many controversies. Consequently, the development of the general rules for understanding the chemistry in ionic liquids is crucial.

A fundamental property of solvent is its solvating power, for instance towards the proton. Therefore, we are interested to investigate the acid-base properties in ionic liquids in order to ultimately find a correlation with the acidic catalysis activity. We then have proposed a colorimetric method to determine the acidity levels accessible in these new media: the Hammett acidity function H0. This spectroscopic method is based on the protonation equilibrium for a family of coloured indicator with pKa’s assumed as solvent independent (following the Hammett proposition).

This presentation will summarize our Hammett acidity measurements in several ionic liquids.
- At first, we will show that it is possible to evaluate the Hammett acidity function with two different coloured indicators, in the same ionic liquid. Since the Hammett acidity of a given mixture was found to depend on the choosen coloured indicator, this suggests the formation of ions associations in ionic liquids. As a result, the ionic liquids are clearly not as dissociating as initially thought and the Hammett acidity function is in fact an apparent function, underestimating the real acidity level.
- The apparent acidity functions have then been compared for several ionic liquids to which an acid has been added ([BMIm][NTf2], [BMIm][BF4], [BMIm][OTf], [BHIm][NTf2], [BMIm][PF6], [HNEt3][NTf2]). The conclusions are as follows: 1) the accessible acidity level is not influenced by the nature of the cation; 2) on the contrary, the nature of anion is very critical and the solvating power towards the proton follows the order: OTf- > NTf2- > BF4- > PF6-. The more the proton is solvated, the less it is acidic.
- Finally, the difference of acidity of two acids, HOTf and HNTf2 (both strong acids in water), has been investigated in [BMIm][BF4], [BMIm][NTf2] and [BMIm][OTf]. In [BMIm][OTf], these two acids show the same acidity (they behave as strong acids) due to the solvent levelling effect; on the other hand, in [BMIm][NTf2] and [BMIm][BF4] allowing higher acidity levels, HNTf2 is stronger than HOTf . The observed difference is also another indication of the lower proton solvation in [BMIm][BF4] or [BMIm][NTf2] versus that in [BMIm][OTf].