Room temperature ionic liquids (RTILs) have been shown as alternative solvents for greener organic reactions and acid catalyzed processes [1]. High levels of acidity can be reached in acidified ionic liquids [2]. The Brønsted acidity has been first evaluated based on the determination of the Hammett acidity function H₀ using UV-Vis spectroscopy in the presence of the adequate indicator [3]. Nevertheless, the uncertainty on the pK_a values of the color indicators seems to be solvent dependent. In fact, it has been observed that two different indicators can lead to significant results deviation which may require additional corrections [4]. As a consequence, a novel approach for the assessment of the acidity in RTILs is presented, being based on the R₀(H⁺) Strehlow function and using the Fc⁺/Fc as an internal reference couple. Protonation features of the addition of p-benzoquinone to an acidified ionic liquid medium is presented. Fig.1 shows the recorded CV redox behavior of p-benzoquinone in BMImBF₄ (1-butyl-3-methylimidazolium tetrafluoroborate), along with Fc⁺/Fc couple without any interference. Hence, the reversible redox waves can be followed for both entities upon the increase of the acidity level. The electrochemical pathways will be addressed, first in a “neutral” and in a gradually acidified media. The results will be discussed and compared to other aprotic solvents, along useful aspects of quinones for the emerging applications (e.g. Li-ion and redox flow batteries).

Acknowledgements

The IFP (Institut Français du Pétrole) is acknowledged for financial support.

References

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[2] C. Thomazeau, H. Olivier-Bourbigou, L. Magna, S. Luts, B. Gilbert, Determination of an Acidic Scale in Room Temperature Ionic Liquids, J. Am. Chem. Soc., 125 (2003) 5264-5265.

[3] T. Robert, L. Magna, H. Olivier-Bourbigou, B. Gilbert, A Comparison of the Acidity Levels in Room-Temperature Ionic Liquids, J. Electrochem. Soc., 156 (2009) F115-F121.

[4] T. Robert, H. Olivier-Bourbigou, L. Magna, B. Gilbert, About the Acidity Level in Room Temperature Ionic Liquids, ECS Transactions, 3 (2007) 71-82.