Standard pKa Scales of Carbon-Centered Indicator Acids in Ionic Liquids: Effect of Media and Structural Implication

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• 作者：Hui Deng ; Xin Li ; Yuan Chu ; Jiaqi He ; Jin-Pei Cheng
• 刊名：The Journal of Organic Chemistry
• 出版年：2012
• 出版时间：September 7, 2012
• 年：2012
• 卷：77
• 期：17
• 页码：7291-7298
• 全文大小：281K
• 年卷期：v.77,no.17(September 7, 2012)
• ISSN：1520-6904

文摘

Energetics of bond dissociation, especially the R鈥揌 bond heterolysis free energy (pK_a), has played a central role in promoting chemistry to become a rational science. Despite the oceans of acidity studies in the literature, the current knowledge is limited to that in the classical molecular solvents and is unable to be extended to anticipate the acidity changes in ionic media. As the latter is now very popular for replacing volatile organic solvents, it becomes highly desirable to know how the driving force of bond cleavage is varied as the medium composition is switched from neutral molecules to the charged ions. Here we describe a general approach to measure absolute pK_a鈥檚 in pure ionic liquid (IL). The standard conditions warranting accurate measurement were outlined. The pK_a鈥檚 of the selected 18 C鈥揌 type indicator acids in four ILs were determined and a convenient indicator platform was constructed for easy expansion of acidity scales. These absolute pK_a鈥檚 make possible, for the first time, direct comparisons of bond energies in IL with those in molecular solvent and in the gas phase and should be able to serve as the standard parameters for calibrating computational methods suitable for the studies in ionic media. The effect of cation and anion in IL in relation to structure was analyzed.