Synthesis of Ionic Liquids

The number of available ionic liquids continues to grow at an ever increasing rate, however only few are used by the wider community. At the same time, it has become fashionable to label charged compounds that not long ago would have been (appropriately) described as sticky oil, as ionic liquids, and a more considerate use of the term would sometimes be desirable. 

The majority of the ionic liquids in use are prepared via salt metathesis reactions and one of the greatest challenges in the field of ionic liquids concerns their synthesis in high purity. Until ionic liquids of well-defined purity 

Numerous examples report on the beneficial effects when catalysis is performed in an ionic liquid, be it increased rate, selectivity, recycling potential or even the accessibility of products that were not available otherwise. Are all these results to be questioned due to insufficiently purified ionic liquids Certainly not, rather the contrary is true. While ionic liquids of poor quality are likely to afford inferior results, it is only in few cases that the contaminant can be held responsible for any observed catalytic activity. An example are acid-catalysed reactions where HF from degradation of the anion could act as the catalyst. 

while the absolute value of reported turnover frequencies and selectivities are likely to be subject to change, it appears that with increasing solvent quality results will change for the better, not the worse. Publications, 

A similar method can be used for the preparation of hexafluorophosphate liquids, but the [PF6] anion is prone to decomposition with the release of HF and care must be taken to avoid this problem. 

In some cases only the first step is required, as with the formation of ethylam-monium nitrate. In many cases the desired cation is commercially available at reasonable cost, most commonly as a halide salt, thus requiring only the anion exchange reaction. Examples of these are the symmetrical tetraalkylammonium salts and trialkylsulfonium iodide. 

This chapter will concentrate on the preparation of ionic liquids based on 1,3-dialkylimidazolium cations, as these have dominated the area over the last twenty 

Ion exchange resin synthesis paths for the preparation of ionic liquids (adapted from 

Although the synthesis appears to be very straightforward, it is actually very difficult to obtain a colourless liquid, as trace amounts of impurities lead to discolouration. The first step of the reaction does not go to completion and the 

Once the two salts are mixed in solution (acetone is a common solvent for this), the sodium chloride precipitates and is removed by filtration. The solvent is then removed under reduced pressure and, since salts have no vapour pressure, the ionic liquid remains in the flask. The problem with this reaction is that it is almost impossible to remove the last traces of chloride ions. The chloride not only influences the physical properties of the liquid such as melting point and viscosity, but is also a good nucleophile and can deactivate catalysts and affect reproducibility. A great deal of effort has been directed towards removal of the chloride contamination, including washes and chromatography, but none have proved to be completely effective [9], This has led to the development of some alternative synthetic routes. Simply exchanging Na[BF4] 

Tom Beyersdorff, ThomasJ. S. Schubert, Urs Welz-Biermann, Will Pitner, Andrew P. Abbott, KatyJ. McKenzie, and Karl S. Ryder 

Ionic liquids (IL) are a new class of salt-like materials that are entirely composed of ions and that are liquid at unusually low temperatures. For the most commonly used definition of the term ionic liquid the boiling point of water was chosen as a reference point, most likely for emotional reasons The term ionic liquids refers to compounds consisting entirely of ions and existing in the liquid state below 100 °C. In many cases the melting point is even below room temperature. 

The history of ionic liquids began with the synthesis of ethylammonium nitrate reported in 1914 by Walden [1]. This material is probably the first described in the literature that fulfills the definition of ionic liquids used today. In this context it 

Electrodeposition from Ionic Liquids. Edited by F. Endres, D. MacFarlane, A. Abbott Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31565-9 

A major breakthrough was achieved in 1951 with the report of Hurley and Wier. They noticed that a mixture of N-ethylpyridinium bromide (EtPyBr) and AICI3 with a eutectic composition of 1 2 X(AlCh) = 0.66 h of EtPyBr to AICI3 became liquid at unusually low temperatures [2], They investigated these melts with regard to their potential use in the electrodeposition of aluminum at ambient temperature [3]. Several studies were carried out on this system, however, its use was very limited since it is only liquid at a mole fraction of X(A1C13) = 0.66 and the ease of oxidation of the bromide ion limits the electrochemical stability. In the following years the main interest in ionic liquids was focused on electrochemical applications [4—6]. 

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The synthesis of ionic liquids can generally be split into two sections the formation of the desired cation, and anion exchange where necessary to form the desired product (demonstrated for ammonium salts in Scheme 2.1-1). 

It is hoped that this section will give the reader a better appreciation of the range of ionic liquids that have already been prepared, as well as a summary of the main techniques involved and the potential pitfalls. While the basic chemistry involved is relatively straightforward, the preparation of ionic liquids of known purity may be less easily achieved, and it is hoped that the ideas given here may be of assistance to the reader. It should also be noted that many of the more widely used ionic liquids are now commercially available from a range of suppliers, including some specializing in the synthesis of ionic liquids [53]. 

The choice of reaction solvent is also of concern in the synthesis of new TSILs. Toluene and acetonitrile are the most widely used solvents, the choice in any given synthesis being dictated by the relative solubilities of the starting materials and products. The use of volatile organic solvents in the synthesis of ionic liquids is decidedly the least green aspect of their chemistry. Notably, recent developments in the area of the solventless synthesis of ionic liquids promise to improve this situation [10]. 

Hu, S., Wang, A., Li, X., Wang, Y., and Lowe, H. (2010) Hydrothermal synthesis of ionic liquid [Bmim]OH-modified Ti02 nanoparticles with enhanced photocatalytic activity under visible light. Chemistry - An Asian Journal, 5 (5), 1171-1177. 

A variety of related microwave-promoted N-alkylations involving more elaborate heterocyclic scaffolds are summarized in Scheme 6.116 [230-234]. Additional examples concerning the synthesis of ionic liquids can be found in Section 4.3.3.2. 

Organic Synthesis Using Microwaves and Supported Reagents 6.2.7.6 Solvent-free Synthesis of Ionic Liquids... 

The synthesis of ionic liquids with BF4 and PF6 as cations has been the subject of much research since they are the most widely used in catalysis. However, it is difficult to make these ionic liquids in a pure form. The original route used to prepare ionic liquids with these anions consists of a metathesis (anion-cation exchange) reaction in which the imidazolium chloride is reacted with the sodium salt of the anion in a suitable solvent [8], The reaction is illustrated in Scheme 4.2 for the tetrafluoroborate salt. 

Wang, Z., et al., The synthesis of ionic-liquid-functionalized multiwalled carbon nanotubes decorated with highly dispersedAu nanoparticles and their use in oxygen reduction by electrocatalysis. Carbon, 2008. 46(13) p. 1687-1692. 

In 2003 Handy and coworkers [65] described the first synthesis of ionic liquids from naturally occurring sugars. They exploited the transformation of D-fructose (1) into monosubstituted imidazoles developed more than 50 years ago by Trotter and Darby [66], to prepare 2 in 61% yield (Scheme 1). 

Makote, R. D., Luo, H. M., Dai, S., Synthesis of ionic liquid and silica composites doped with dicyclohexyl-18-crown-6 for sequestration of metal ions. In Clean Solvents, ACS Symposium Series 819, 26-33, 2002. 

Scheme 11 Synthesis of ionic liquids via solvent-free alkylation of N-methyl imidazole [63]...

The German public funded project NEMESIS focuses on the design and development of microreactors for the synthesis of ionic liquids at pilot scale [52], Scientific objectives are to increase the yield of the corresponding ionic liquid as well as to decrease reaction time from hours up to days currently. Ionic liquids, a new innovative class of materials, are synthesized using microreaction technology. Possible application fields are their use as electrolytes for the elaborate deposition of metals. A concept for regeneration of the electrolyte is also considered. 

Fig. 2.1 Examples of cations commonly used for the synthesis of ionic liquids...

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