How to Choose Ionic Liquids

Conventional organic solvents can be placed roughly in one of the following four categories [43], even if more refined versions of this scheme are available (a) apolar aprotic (e.g., cyclohexane) (b) weakly polar aprotic (e.g., diethyl ether) (c) dipolar aprotic (e.g., acetonitrile, dimethylformamide) (d) protic (e.g., water). 

Where are ionic liquids in this classification Except for nonpolar groups, we can say that with a proper choice of the anion and the cation, ionic liquids could fit into any of these categories. If we consider the large number of possible anion-cation combinations (see Table 1), the selection of the IL is far from being obvious and a series of problems have to be overcome before development of industrial processes (cf Section 5.1). Some of the properties have to be known when ILs are used for catalysis. 

While their nonvolatile character is a major advantage over conventional solvents, it is a limitation due to the impossibility of purifying ILs by distillation. Pmification can then be a significant challenge. 

Traces of halide impurities (Cr, F, or even other anionic impurities) are often present in ionic liquids, notably when they are obtained by an anion exchange reaction. Most of the time, this presence of halides greatly influences the comse of the catalytic reaction. For example, a detrimental effect of chlorides was observed for hydrogenation or Michael addition [44,45], while a beneficial effect was reported for the Heck reaction [46]. It is therefore necessary to measure carefully the amount of halide impurity in ILs, and this amount can be quantified by high-performance ionic chromatography [47]. This precaution is especially important for colored commercial ionic liquids, which are likely to contain a significant amount of halide impurity as well as of other anions. 

AU ILs, either hydrophobic or hydrophilic, are hygroscopic and quickly absorb a significant quantity of water when exposed to air. Their properties, such as viscosity, solubility, polarity, conductivity, and electrochemical characteristics [48] (see Section 5.2.1.2), are significantly dependent on the concentration of water. Wet ionic liquids cannot be considered as homogeneous solvents but rather as nano- 

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