Synthesis of Ionic Liquids Using Microwave Heating

Synthesis of Ionic Liquids Using Microwave Heating 

1-methylimidazole and n-butyl halides [15]. After irradiating N-methylimidazole and n-butyl bromide for 2 min in the domestic oven, l-butyl-3-methylimidazolmm bromide [BMIMjBr was obtained in 78% yield. When the ionic liquid starts to form, the polarity of the reaction medium increases and, therefore, microwave absorption increases. The observed reactivity order was I Br Cl. The yields of ionic liquid improved when sequential irradiation was used. For example, irradiation for 30 s then 3 x 15 s irradiation intervals is better than one period of 75 s. 

This method is not only limited to halide-based ionic liquids. Synthesis of ionic liquids containing a tetrafluoroborate anion and imidazolium cation has been performed successfully [16] by use of a modified domestic microwave oven with pulsed irradiation (5 x 30 s). Approximately 90% yields of the desired ionic liquids were usually obtained compared with 36% after the same reaction time using conventional heating. Microwave irradiation has been used in the synthesis of l-ethyl-3-methylimidazolium benzoate and dialkyl imidazolium tetrachloroaluminate ionic liquids [17, 18]. These reactions were again performed in a domestic microwave oven using pulsed irradiation. 

Although reports of ionic liquid synthesis in domestic microwave ovens are promising, synthesis in these systems has several disadvantages. For chloroalumi-nates there were problems with the reproducibility of the synthesis. In domestic microwave ovens, control of the reaction conditions is difficult and overheating of the reaction mixtures can easily occur. A modified procedure was published, again using a domestic oven, but placing the reaction vessel in a beaker of water [19]. 

This water moderation reduced the amount of irradiation reaching the sample and thus reduced the effects of overheating. It also functioned as a heat sink. Other problems with domestic ovens in the synthesis of ionic liquids are that volatile alkyl halides are released inside microwave cavity, which can be hazardous, and large-scale synthesis is difficult because of poor control, possible contamination, and product decomposition. 

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