The application of ionic liquids as solvents is of interest due to their unusual and potent solvent characteristics. They are salts that are liquids at ambient temperature and typically consist of an organic cation and an inorganic anion. Ionic liquids are very polar and are good solvents for a wide range of organic, inorganic and polymeric compounds. Their properties are similar to many other high boiling, polar solvents for example dipolar aprotics. One application they are used for is to dissolve materials that are typically very difficult to dissolve e.g. lignin. A wide variety of structures are available and the anion and cation can be fine-tuned to give a wide range of solvent properties (e.g. hydrophilic, hydrophobic). A wide range of reactions have already demonstrated in this medium, including: Diels-Alder reactions; Alkylation reactions; Hydroformylation reactions; Friedel Crafts reactions; Pd-mediated C-C bond formation; Alkene polymerisation and biotransformations.
The key reason for which they are quoted as being ‘green’ is that they have very low vapour pressure and hence are non-volatile. However this is of course a narrow viewpoint focussing on only one parameter, and other factors should be taken into consideration, including their synthesis and end-of-life considerations. (For more information on this see Topic 2 Metrics). Due to their low vapour pressures they cannot be distilled, making them difficult to purify. They are also typically expensive to buy and often other solvents are used in their preparation or purification. Some of the reasons for their limited uptake within the pharmaceutical industry have been the lack of ecotoxicity and life cycle impact data, cost and end of life issues (recycling or disposal). There is also concern surrounding the environmental fate and toxicity of some ionic liquids.
Green Solvents, Ionic Liquids, Handbook of Green Chemistry (ed. ) vol. 6, Wiley, 2014.,
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