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An Acidity Scale of 1,3-Dialkylimidazolium Salts in Dimethyl Sulfoxide Solution

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journal contribution
posted on 28.09.2007, 00:00 by Yuan Chu, Hui Deng, Jin-Pei Cheng
Equilibrium acidities of 16 1,3-dialkylimidazolium-type ionic liquid (IL) molecules (116) were systematically measured by the overlapping indicator method at 25 °C in dimethyl sulfoxide (DMSO) solution. The pKa values were observed to range from 23.4 for IL 12 to 19.7 for IL 6 (Tables and ), responding mainly to structural variations on the cation moiety. Excellent agreement between the spectrophotometrically determined pKa and that derived from NMR titration for 1,3,4,5-tetramethylimidazolium bis(trifluoromethanesulfonyl)imide (12) and the close match of the obtained pK values with the reported data in literature provide credence to the acidity measurements of the present work. The substituent effects at the imidazolium ring and the effects of counterions on the acidities of ionic liquids are discussed.