Standard p<i>K</i><sub>a</sub> Scales of Carbon-Centered Indicator Acids in Ionic Liquids:
Effect of Media and Structural Implication
- Publication date
- Publisher
Abstract
Energetics of bond dissociation, especially the R–H
bond heterolysis free energy (p<i>K</i><sub>a</sub>), 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 <i>molecules</i> to the charged <i>ions</i>. Here we describe a general approach to measure absolute p<i>K</i><sub>a</sub>’s in pure ionic liquid (IL). The standard
conditions warranting accurate measurement were outlined. The p<i>K</i><sub>a</sub>’s of the selected 18 C–H type
indicator acids in four ILs were determined and a convenient indicator
platform was constructed for easy expansion of acidity scales. These
absolute p<i>K</i><sub>a</sub>’s 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