Alternative Synthesis and Industrial Production

For academic researchers, it is possible and even desirable to try to obtain perfectly pure ILs, with negligible amounts of impurities. However, such achievement of purity would prevent the possible production of I Ls on a large scale at a reasonable cost. Scientists in the industrial field have then to determine the specification in terms of purity which is required for each application and the corresponding production method. 

Moreover, ILs are sometimes described as green solvents. However, the classical two-step synthesis uses a large amount of solvent and produces important byproducts. Some effort is therefore necessary to achieve simpler and greener synthesis of ILs. This situation has led to the development of some alternative syntheses which are likely to solve some of these difficulties. 

The potential detrimental effect of halide has been described previously. A first alternative to the two-step synthesis is the direct alkylation of N-alkylimidazole. H owever, the range of available alkylating reagents is limited and they are sometimes expensive. Two-step syntheses of ILs which do not use imidazolium halide as an intermediate were then developed. For example, Seddon described the reaction of N-alkylimidazole with ethyl trifluoroacetate, which leads to imidazolium trifluoro- 

Another method has been described for the synthesis of halide-free ILs which uses an imidazolylidene carbene as an intermediate. The starting material is an imidazolium halide which is treated with sodium tert-butoxide to form an imidazolylidene carbene. The latter is distilled in a Kugelrohr apparatus, then reacted with a Br0nsted acid. In this way, it is possible to obtain an IL free from halide contamination, but traces of N-alkylimidazole or acid may be present. 

Some efforts have been devoted to the improvement of the classical syntheses. Indeed, the quaternization of N-alkylimidazole with alkyl halide via the conventional heating method in refluxing solvent requires several hours to afford reasonable yields and also uses a large excess of alkyl halides/organic solvent as the reaction medium. Similarly, the metathesis reactions are lengthy and generally result in reaction products which require purification. 

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