Room-temperature ionic liquids have been developed over the past decade as
green solvents for industrial applications, ' ranging from the petrochemical industry, via heavy chemicals, fine chemicals, agrochemicals, and pharmaceuti-
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cals, to the nuclear industry, solar cells, and electrochemistry. ' Recent inde-
pendent reports, , ' and many reviews, , 9 have highlighted ionic liquids as representing a state-of-the-art, innovative approach to green chemistry, and they are even suitable media for preparing and studying nanoclusters.30 The potential of ionic liquids to act as solvents for a broad spectrum of chemical processes is attracting increasing attention from industry, because these processes promise sig-
nificant environmental benefits. , Prof. Paul Anastas, former director of the ACS Green Chemistry Institute,32 ex-chief of the Industrial Chemistry Branch at the Environmental Protection Agency in Washington, D.C., and advisor to both President Clinton and President Bush, is convinced that ionic liquids can contribute significantly to sustainable chemistry and the development of green technology. He was reported in Chemical & Engineering News as stating:33
Green chemistry aims to design the hazards out of chemical products and processes, including solvents. With ionic liquids, you do not have the same concerns as you have with, for example, volatile organic solvents, which can contribute to air pollution. Ionic liquid chemistry is a very new area that is not only extremely interesting from a fundamental chemistry point of view, but could also have a very large impact on industry.
Thus, room-temperature ionic liquids have the potential to provide environmentally friendly solvents for the chemical and pharmaceutical industries. The ionic liquid environment is very different from normal polar and nonpolar organic solvents; both the thermodynamics and the kinetics of chemical reactions are different, and so the outcome of a reaction may also be different. Organic reactions that have been successfully studied in ionic liquids include Friedel-Crafts,34
Diels-Alder, Heck catalysis, chlorination, ' enzyme catalysis, , 9 polymerization,40 cracking,41 oxidation,42 and hydrogenation.43 Indeed, a comprehensive review of Heck chemistry44 concluded that "the ionic liquid process appears as one of the cleanest recyclable procedures so far described for the Heck reaction."
One of the suggestions to arise out of this deluge of new chemistry is that ionic liquids are "designer" solvents.33 For this to be true, there is an implication (implicit rather than explicit) that for every chemical reaction of interest, it should be possible to design, or tailor, a solvent to optimize that reaction. Given that the chemical industry currently uses only about 600 molecular solvents, the claim of designer solvents for ionic liquids must imply that many, many more ionic liquids could be prepared, by orders of magnitude, than are currently available, or indeed imaginable, for conventional solvents. In this chapter, we examine the question, How many ionic liquid systems could there be?
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