Quantifying Acidity in Heterogeneous Systems: Biphasic pKa Values.

Acidities of lipophilic compounds, such as various ligands or catalysts, in systems consisting of an aqueous phase at equilibrium with a water-immiscible phase (lipid bilayers, phase transfer catalysis, sensor membranes, to name just few) are typically approximated by the aqueous pKa values. Our research shows that such approximations can lead to seriously biased estimations of the acidities as the bulk of solvated H+ ions reside in the aqueous phase, while the lipophilic species─both neutral acid and anion─predominantly reside in the organic phase. Therefore, the use of aqueous pKa in such situations is not justified. In this work, we provide a more accurate description of the acidities of acids in such systems by applying the biphasic pKa concept. Biphasic pKa values (pKaow values) of 35 acids of various structures and chemical properties were determined in a 1-octanol:water system. We provide detailed descriptions of the UV-vis and NMR measurement methods. The directly obtained (apparent) pKaow values depend on concentration. Concentration-independent values were obtained by extrapolating the apparent values to zero concentration using a Debye-Hückel model.

Validation and extension of the gas-phase superacidity scale.

RATIONALE: In recent years it has become increasingly evident that the previously reported experimental gas-phase acidity (GA) values of several strong acids differ markedly from the corresponding high-level computational values. In this work, the superacidic part of the current gas-phase acidity scale was validated and extended. METHODS: For that, the strongly acidic section of the gas-phase acidity scale was remeasured using the equilibrium Fourier transform ion cyclotron resonance (FTICR-MS) method, adding new compounds and introducing methodological changes. In particular, a novel approach for anchoring the scale was used - the results were anchored to the computational (W1BD) GA values of trifluoromethanesulfonic acid and bis(fluorosulfonyl)imide (291.3 and 286.2 kcal mol-1 , respectively). RESULTS: The newly measured section consists of 20 gas-phase superacids and its consistency standard deviation is 0.2 kcal mol-1 , indicating good consistency. In contrast to the previously reported experimental gas-phase acidities for a number of important superacids, the current results are consistent with high-level theoretical GA values. Structure-acidity relationships based on the current results as well as available MeCN and DCE acidity data were described and explained. CONCLUSIONS: The introduced methodological innovations were found to be adequate and strong evidence is presented in support of the current GA values of the strong acids.