One-pot and Solventless Synthesis of Ionic Liquids under Ultrasonic Irradiation

Abstract

A novel method is described for the one-pot synthesis of various ionic liquids in a competitive time. By using ultrasonic irradiation, different families of nitrogen-bearing ionic liquids can be obtained in a solvent-free or in aqueous medium, which gives a greener touch to the overall process.

Key words

ionic liquids - ultrasound - solvent-free reaction

References and Notes

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15

The used ultrasonic probe was a Branson 20 kHz digital sonifer or a Ultrasons Annemasse S.A. 30 kHz probe. Acoustic powers were determined by calorimetry. [17] ¹H and ¹³C NMR spectra were recorded on a Bruker AMX 200 MHz spectrometer. IR analyses were performed on an ATI Mattson Genesis Series FTIR instrument. All precursor products are commercially available and were used without further purification. Potassium salts (1 equiv), alkyl bromide (1 equiv) and nitrogen-bearing heterocycles were introduced in a 25-mL cooling-jacket glass reactor. H₂O was added when 1-bromobutane was used as alkyl bromide. The medium was sonicated at 20 kHz or 30 kHz for the times and at the temperatures and acoustic powers listed in Tables [1] and [3] -5. A heat-conducting fluid was used in the cooling jacket to maintain the overall temperature at about 80 °C during the experiments. The final mixture was then poured into acetone and filtered through Celite. Acetone was removed under vacuum. Hydrophobic ILs were washed with H₂O (4 × 20 mL) and Et₂O (4 × 20 mL), to be finally dried at 90 °C under vacuum (3 h). Hydrophilic ILs were dissolved in H₂O (50 mL) and extracted with CH₂Cl₂ (4 × 25 mL); solvents were removed under vacuum and RTILs are washed with Et₂O (4 × 20 mL) to be finally dried at 90 °C under vacuum (3 h). All products were checked by ¹H and ¹³C NMR spectroscopy and no organic impurity was observed. No significant trace of H₂O was observed in the IR spectra.