onic liquids (ILs) as solvents became a hot topic in the late 1990s, with the focus on the novel ways they could be used. Information about the ILs themselves was scattered throughout the literature until a group of researchers, led by Robin Rogers and based in the Center for Green Manufacturing at the University of Alabama, Tuscaloosa, published paper #5. In this the authors reported how these solvents could be made, itemized their various properties, and discussed the precautions to be taken when using them. Not surprisingly this useful paper now finds itself being heavily cited, and one of its citers is paper #6.

There is a growing need for new solvents to replace the traditional ones whose volatility is seen as a threat to the global environment. Salts are ionic in nature, and because of this they tend to be high melting solids, and while they may be non-volatile, this excludes them from organic syntheses. However, some salts have low melting points and are liquids even at room temperatures, and chemical companies are starting to investigate them as solvents.

The 1-alkyl-3-methylimidazolium salts are potentially the most interesting, exhibiting unexpected solvent properties and behavior which vary according to the nature of the alkyl group and the anion. Those in #5 have n-butyl, n-hexyl, and n-octyl chains attached to the five-membered cationic ring, together with anions that are either hydrophilic (e.g., chloride and iodide) or hydrophobic (e.g., hexafluorophosphate). The water content, density, viscosity, surface tension, melting point, and thermal stability of these were investigated and tabulated. As might be expected, the longer the carbon chain, the more "organic" was the solvent, but it was the anions that really determined their behavior. Perhaps the most remarkable IL is n-hexyl-3-methylimidazolium hexafluorophosphate, which is liquid down to minus 61° C.

More recently the Alabama group has been investigating ILs with unique applications, and one of their most publicized discoveries was the ability to dissolve cellulose (see R.P. Swatloski, et al., J. Am. Chem. Soc., 124[18]: 4974-5, 2002). They have also looked at mixtures of ILs and water (see K.E. Gutowski, et al., J. Am. Chem. Soc., 125[22]: 6632-3, 2003), clearly of importance if these solvents are to find wide application. Paper #5 also discussed how these ILs behave with water; some are entirely miscible while others give separate phases. Traces of water can have a disproportionate effect, and Rogers points out the difficulties of drying ILs, cautioning would-be users to be aware of this.

However, this was not the only difficulty facing the authors of paper #6, which comes from McGill University Chemistry Department, Montreal, Canada, and reports the use of ILs in a series of reactions in which lipase enzymes are used to catalyze the acetylation of 1-phenylethanol. Researchers Romas Kazlauskas and Seongsoon Park used imidazolium ILs and found that these solvents deactivated the lipases, but not if they were pre-treated with aqueous sodium carbonate. Subsequent reactions in ILs were then just as rapid and, equally important, were just as enantioselective as in a normal organic solvent like toluene. One particular reaction showed how beneficial ILs could be: this was the acetylation of glucose catalyzed by lipase B from Candida Antarctica, which was even more selective that in a normal organic solvent such as acetone. Moreover glucose can be up to 100 times more soluble in ILs than in conventional solvents.

Commenting on the growing use of these solvents, Steve Ley of Cambridge University tells Science Watch: "ILs are beginning to be recognized in organic synthesis because they allow transformations that we did not think were possible." Part of this remarkable growth followed Ley’s discovery that microwave heating of ILs greatly enhanced the rates of reactions (see S.V. Ley, et al., J. Chem. Soc., Perkin Trans. 1, 4:358-61, 2001). Rogers is also enthusiastic about the future of the ILs: "Almost everything we do leads to a new area of research where we have so much to learn of the fundamentals of the chemistry, while at the same time having so many possible applications to explore." Nor has he lost sight of what these solvents have to offer environmentally, although it is their fascinating chemistry that lures him on: "While the overall motivation is to develop green technologies for industrial practice, one can’t help but get caught up in the wonder of the unknown and trying to tease out that fundamental picture of what they are and how they work."

It’s an IL wind that looks set to blow everyone some good.