Investigation of the acidity and nucleophilicity of ionic liquid mixtures and their effects on reaction rates

The Hammett acidity functions (H0) of mixtures of 1-butyl-3-methylimidazolium hydrogensulfate with sulfuric acid, and 1-butylimidazolium hydrogensulfate with sulfuric acid, were measured using UV dyes. A linear correlation between H0 value and the concentration of sulfuric acid was observed for both mixtures. H0 values of the mixtures were also measured using an NMR probe, mesityl oxide. The H0 values calculated from the mesityl oxide experiments agreed well with the H0 values calculated from the UV dye data. 1-Butyl-3-methylimidazolium hydrogensulfate, 1-butylimidazolium hydrogensulfate, 1-butyl-3-methylimidazolium chloride, 1-butylimidazolium chloride, 1-butylimidazolium dihydrogenphosphate and 1-butyl- 3-ethylimidazolium acetate were examined as both solvent and nucleophile in nucleophilic substitution reactions with 4-methylbenzyl alcohol. Only 1-butyl-3-ethylimidazolium acetate failed to react. The mechanism of the nucleophilic substitution reactions of 1-butylimidazolium hydrogensulfate and 1-butyl-3-methylimidazolium chloride were investigated using Hammett plots. All benzyl alcohols substituted via the SN2 mechanism in 1-butyl-3-methylimidazolium chloride, however both the SN2 and SN1 mechanisms were seen in 1-butylimidazolium hydrogensulfate. Finally, the structures of hydrogendichloride ionic liquids ([HCl2]−) were investigated using infrared spectroscopy.Open Acces