In-volatile solvent ionic liquids

As with all solvent alternatives discussed in this book, palladium catalysed C-C bond-forming reactions in RTILs have been studied at length. Because of the low volatility of ionic liquids and their rapid dielectric heating upon microwave irradiation, reaction times for Heck couplings have been significantly reduced by combining the two technologies. ... 

Among the new synthetic methodologies recently studied and exploited, the use of ionic liquids as solvent-catalysts to perform more environmentally friendly Friedel-Crafts acylations has been developed. In contrast to volatile organic solvents, ionic liquids have no measurable vapor pressure, and, therefore, fhere is no loss of solvent through evaporation. Moreover, ionic liquids can be easily recovered, cleaned, and reused for different runs. 

Although more research needs to be conducted on IL toxicity and environmental impact, both solvents are considered benign, and the need to use volatile organic compounds to separate reaction products may be eliminated. It is, in principle, possible to run a homogeneously catalyzed reaction in continuous mode. Ionic liquid and immobilized catalyst remain in the reactor at all times, which makes their recycling possible. That is very important, because they tend to be rather expensi-... 

The use of microwave ovens offers a solution to reducing the reaction time of organic transformations and leads to an increased yield compared with traditional methods. However, because the reaction medium is quickly heated to high temperatures, problems can occur as a result of an increase in the internal pressure in sealed vessels. Ionic liquids have proved to be an efficient aid for microwave heating. They can be heated for an extended period without signs of decomposition or an increase in pressure. Furthermore, when a small amount of ionic liquid is added to a volatile organic solvent, it can be heated to well above its boiling point. 

Ionic liquids have many useful properties. Unlike most molecular liquids, they are nonvolatile (that is, they don t evaporate readily) and nonflammable. They tend to remain in the liquid state at temperatures up to about 400 °C. Most molecular substances are liquids only at much lower temperatures, 100 °C or less in most cases (see Table 11.3). Because ionic liquids are good solvents for a wide range of substances, ionic liquids can be used for a variety of reactions and separations. These properties make them attractive replacements for volatile organic solvents in many industrial processes. Relative to traditional organic solvents, ionic liquids offer the promise of reduced volumes, safer handling, and easier reuse, thereby reducing the environmental impact of industrial chemical processes. 

The low volatility of ionic liquids and the easy separation of catalysts (which usually remain in these polar media) have made ionic liquids an interesting alternative to typically used organic solvents. Rather unsatisfactory results have been obtained in both copper-mediated [36] and copper-free [37] Sonogashira reactions, with aryl iodides being the only aromatic electrophiles coupled at reaction temperatures between 60 and 80 °C. It should further be noted that imidazolium-based ionic liquids are not necessarily innocent solvents, but can be deprotonated in the presence of bases to generate N-heterocycUc carbenes (NHCs). 

Lagrost C, Carrie D, Vaultier M, Hapiot P (2003) Reactivities of some electrogenerated organic cation radicals in room-temperature ionic liquids toward an alternative to volatile organic solvents J Phys Chem A 107 745-752... 

During the last decade, ionic liquids have been transformed from poorly understood materials to the focus of many research activities, both in academic research and industrial applications A huge interest in using ionic liquids as an alternative medium for CO2 capture has become apparent because of its potential advantages compared to other conventional solvents, such as MEA There are many ways in which an ionic liquid can be defined, and perhaps the most widely accepted definition is, A material that is composed solely of ions . Ionic liquids are a very versatile class of solvents, due to their unique characteristics, such as the ability to manipulate and tune their physicochemical properties through cation or anion selection, non-volatility under ambient conditions, high thermal stability, as well as high CO2 solubility, may overcome many of the problems associated with current C02-removal techniques It is also very important to note that ionic liquids are not solutions of ions in water, or other solvents. 

In reactions 9.90 and 9.91, the precursor is the di(trifluoro-methylsulfonyl)amido salt of the metal rather than a halide as in eqs. 9.88 and 9.89. If M3 is reacted with [BMpyr][N(S02CF3)2] (compare with eq. 9.90), the product is [BMpyr]4[Prl6][N(S02CF3)2] containing the octahedral ion and a non-coordinated [N(S02CF3)2] ion. Separation of products from ionic liquid solvents is facile if the product is insoluble (separation is by filtration or removal of the solvent by cannula). However, if the product is soluble, separation may be difficult since the low volatility of ionic liquids prevents their ready evaporation. If the product is volatile, it can be separated by distillation because the ionic liquid has such a low vapour pressure. 

In recent years, ionic liquids (ILs) have attracted much attention [5-8]. In comparison with the conventional solvents (usually water and organic solvents), ILs have some unique properties. For example, ILs are an interesting class of tunable and designable solvents with essentially zero volatility, wide electrochemical window, nonflammability, high thermal stability, and wide liquid range. These make them unique for uses and applications in the areas of chemistry and chemical engineering. 

In recent years, ionic liquids have emerged as possible "green solvents", that is environmentally benign substances mainly because they have negligible vapor pressure (Liu et al., 2005). One of the primary driving forces behind research into ionic liquids is the perceived benefit of substituting traditional industrial solvents, most of which are volatile organic compounds (VOCs), with nonvolatile ionic liquids. Replacement of conventional solvents by ionic liquids would prevent the emission of VOCs, a major source of environmental pollution (Polshettiwar Varma, 2008). 

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