Spectroscopic determination of relative Brønsted acidity as a predictor of reactivity in aprotic ionic liquids

Abstract

A simple titration technique utilizing pyridine as a FTIR spectroscopy probe is demonstrated to successfully predict relative Brønsted acid-limited reaction rates in different ionic liquid (IL) environments. Relative acidity is shown to vary across three aprotic ILs in a manner that is specific to the particular acid-IL pairing.

Interplay of Acidity and Ionic Liquid Structure on the Outcome of a Heterocyclic Rearrangement Reaction

“…These findings agree with previously reported observations about the acidity levels in IL solutions of strong acids . However, the rather complex influence of several concomitant factors on the observed acidity in ILs, including interactions between the conjugated base and the IL cation, as well as the affinity between acidic proton and IL anion has been previously reported …”

“…36 However, the rather complex influence of several concomitant factors on the observed acidity in ILs, including interactions between the conjugated base and the IL cation, as well as the affinity between acidic proton and IL anion has been previously reported. 37 Kinetic Measurements. All kinetic experiments were conducted at 298.1 K, observing, in all cases, pseudo-firstorder kinetic traces.…”

Validation of pH Standards and Estimation of the Activity Coefficients of Hydrogen and Chloride Ions in an Ionic Liquid, Ethylammonium Nitrate

“…Therefore, a current ongoing debate discusses the actual definition of Δp K a and what exactly it expresses . On the other hand, PILs made from a very weak acid were found to have a surprising ionic property, where hydrogen ions are expected to rarely dissociate according to their Δp K a values. − From this point of view, the direct observation based on solvatochromism, − NMR, , titrimetric p K a determination, − and so on, of the actual hydrogen ion activity is a significant target of current research. Recently, scaling of pH in ionic liquids using a commonly applicable pH scale is addressed. − Analogous investigations were performed previously for a number of conventional nonaqueous solvents; − however, these types of studies on ionic liquids are few and provide inadequate information.…”

Determination of the Hammett Acidity Functions of Triflic Acid/Ionic Liquid Binary Mixtures by the 13C NMR-Probe Method

“…However, in nonaqueous media or concentrated solutions including ILs, determining the p K a may be difficult because of its strong nonideality, and a glass electrode is generally unsuitable for measuring such systems. The alternative approaches, including UV–vis spectroscopy, − infrared or Raman spectroscopy, − and NMR spectroscopy (i.e., 13 C NMR, 31 P NMR, 1 H NMR), − have been applied to investigate the acid strength of solutes in nonaqueous or concentrated solutions. The Hammett acidity functions ( H 0 ) is a thermodynamic quantitative measure of the acid strength of the nonaqueous or concentrated Brønsted acids; here, the acid strength is defined as the extent to which the acid protonates a base of known basicity.…”

“…However, the method has a strong dependence on indicators, which means the indicators must be changed constantly. A high-purity and colorless system is also required, ,, which suggest its application in ILs that are often the colored liquids is limited. The Lewis or Brønsted acid strength of an unknown acid can be determined using acetonitrile or pyridine as a probe molecule by characterizing the probe molecule in the IR spectrum of the acid and determining its peak position in the absorption bands.…”