Rationale And Definitions Why Use Ionic Liquids

Ionic liquids are, quite simply, liquids that are composed entirely of ions.17-22 Thus, molten sodium chloride is an ionic liquid: a solution of sodium chloride in water (a molecular solvent) is an ionic solution. The term ionic liquids was selected with care, as it is our belief that the more commonly used phrase molten salts (or simply melts) is referential, and invokes a flawed image of these solvents as being high-temperature, corrosive, viscous media (cf. molten cryolite). The reality is that room-temperature ionic liquids can be liquid at temperatures as low as — 96°C, and are typically colorless, fluid, and easily handled. To use the term molten salts to describe these novel systems is as archaic as describing a car as a horseless carriage. Moreover, in the patent and recent academic literature, ionic liquids are now taken as being liquids that are composed entirely of ions, which are fluid at, around, or below 100°C. There is nothing sacred about the temperature of 100°C, it is merely a convenient marker.

In this chapter, I have already used a term new to chemistry: neoteric solvents.10 The word neoteric is well established in the English language.5 It is used here to indicate a class of novel solvents that have remarkable new properties, that "break new ground," and that offer a huge potential for industrial application. It is applied not only to ionic liquids, but also to supercritical fluids, another type of solvent that shows huge promise for green synthesis. Thus the term neoteric solvents covers both ionic liquids and supercritical fluids, and brings together under one banner the two most promising solvent systems for cleaning up the modern chemical industry.

Perhaps the most important question to be answered here is: Why would any sane chemist wish to use a room-temperature ionic liquid as a solvent for the study of industrially relevant catalytic process? To address this question, it is worth considering the history of reactive chemistry. For two millennia, chemistry has been studied in water, the solvent that covers 70% of the surface of our planet. In the nineteenth century, the range of known chemistry was dramatically expanded by the use of a wider range of solvents: organic solvents (e.g., alcohols, chlorinated hydrocarbons, arenes, and nitriles) caused a blossoming in the known organic chemistry; liquid ammonia opened up low-oxidation-state chemistry; nitrogen(IV) oxide and bromine(III) fluoride allowed the exploration of high-oxidation-state chemistry; and solvents such as sulfuric acid and liquid hydrogen chloride combined high solvation with acid catalysis. However, for all the huge chemical variety provided by this wide range of chemically disparate solvent systems, they all have one thing in common—they are all molecular solvents. There has been no learning experience from studying reactive chemistry in ionic solvents. Thus, our basic understanding of reactive chemistry is potentially flawed, as it is derived from a biased data set. Will apparently well-understood, conventional reaction types, such as the Friedel-Crafts reaction, follow the mechanisms that we teach undergraduates when studied in an ionic environment? Will the short-lived reaction intermediates found in conventional solvents be long-lived stable intermediates in an ionic liquid? Will changes in the relative stabilities of reaction intermediates change the available reaction pathways? Will the expected changes in reaction kinetics shift the balance between thermodyn-amic and kinetic control, even for well-known, simple reaction types, and hence produce different products? The answers to these questions will only be discovered by extensive experimental studies of these systems, which are now well under way in many laboratories around the world. Of one thing we can be sure, both the thermodynamics and the kinetics of any reaction will be different in an ionic fluid than in a molecular solvent, and so the outcome of the reaction may well be different too, with the choice of ionic liquid dictating the outcome (vide infra).

§Neoteric is defined in the Oxford English Dictionary as meaning: recent, new, modern.

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