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Synthesis of aprotic ionic liquids

Abstract

Ionic liquids (ILs) are typically a salt in the liquid state. They have been intensively investigated for many applications, including as solvents, electrolytes, catalysts, energy storage materials and lubricants, owing to their synthetic flexibility; there is a large number of different possible combinations of anions and cations, which can lead to ILs with very distinct properties. However, there is no standard methodology for the synthesis of ILs, with the quality of these being based solely on the internal experience of each research group. This Primer provides a guide to IL synthetic practices, outlining considerations when choosing the synthetic route as well as evaluating the advantages and disadvantages of each route. The Primer serves as a reference for both new and experienced researchers, to help determine whether the IL from a specific route will be suitable for their intended application, be it ultra-pure, fast or cheap.

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Fig. 1: Common routes leading to symmetrical [C4C4im]+ ionic liquids.
Fig. 2: Common routes leading to [P4441]+ with different anions.
Fig. 3: Direct alkylation of bases to form an ionic liquid.

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Acknowledgements

F.P. acknowledges funding from the President’s Ph.D. scholarship. A.D. thanks GlaxoSmithKline (GSK) and the Engineering and Physical Sciences Research Council (EPSRC) for funding her Ph.D. studies. G.J.S. thanks the National Institute for Health Research (NIHR) for funding his Ph.D. studies. J.B. thanks the Defence Science and Technology Laboratory (Dstl) for funding his Ph.D. studies. F.O. acknowledges funding from the Ministry of Higher Education of Malaysia. Nuclear magnetic resonance (NMR) spectra were recorded at the NMR facility of Imperial College London. The authors thank L. Haigh for performing the mass spectrometry measurements, T. Costantini and M. Shaffer (Imperial College London) for giving access to their ultrasonic processor, and A. Wall and E. Tate (Imperial College London) for giving access to their microwave reactor.

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S.K., A.D., F.P. and T.W. contributed substantially to discussion of the content. All authors researched data for the article. All authors reviewed and/or edited the manuscript before submission.

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Glossary

Aprotic ILs

Ionic liquids (ILs) that do not contain any dissociable hydrogen.

Task-specific ILs

Ionic liquids (ILs) that incorporate functional groups designed to impart to them particular properties or reactivities.

Atom economy

The amount of starting material of a reaction that ends up as useful products.

Debus–Radziszewski reaction

A one-pot chemical reaction leading to the formation of symmetrical dialkylimidazolium ionic liquids (ILs).

Sonochemical synthesis

A chemical synthesis that is guided or accelerated by the irradiation of the reaction vessel with ultrasund.

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Koutsoukos, S., Becker, J., Dobre, A. et al. Synthesis of aprotic ionic liquids. Nat Rev Methods Primers 2, 49 (2022). https://doi.org/10.1038/s43586-022-00129-3

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