Measurements and computations of acidity and basicity of strong and superstrong acids (superacids) and bases in organic solvents is among the core research topics at the UT Chair of Analytical Chemistry. In a recent works (Chem. Sci. 2017, 8, 6964-6973.,    J. Phys. Org. Chem. 2013, 26, 162-170.,    J. Phys. Chem. A 2015, 119, 735–743.,    J. Phys. Chem. A 2016, 120, 3663–3669.) the behavior of a number of acids – ranging from weak to strong and superstrong acids (superacids) was examined in three solvents (water, acetonitrile, 1,2-dichloroethane) and in the gas phase. Acidities (pK_a values) of a number of different acids including the well-known superacids trifluoromethanesulfonic (triflic) acid, bis-trifylimide (Tf₂NH), etc as well as the carborane superacids (closo-carborane superacids), but also weaker acids (HCl, acetic acid, phenol) etc are examined in the above mentioned solvents. pK_a of superacids are not easy to find in literature. Trends of acidity changes on moving from water to the gas phase depnding on the on the nature of the acidity centre and the molecular structure are analyzed. The acidity orders are different in water, MeCN, DCE and the gas phase. In some cases – notably, the hydrohalogenic acids HCl, HBr and HI – the differences are dramatic. These three acids are among the strongest known acids in water but have modest acidity in the gas phase. In contrast, 9‑C₆F₅‑Octafluorofluorene, a weak acid in water (approximately of the strength of phenol) is quite strong acid in the gas phase, beating any of the hydrohalogenic acids.

It is demonstrated that the decisive factor for behavior of the acids when transferring between different media is the extent of charge delocalization in the anion and that the recently introduced WAPS parameter in spite of its simplicity enables interpretation of the trends in the majority of cases. The acidity data together with references to specific publications are collected in the Table below.

Table of pK_a values^a of acids in different solvents.

^a Values from http://dx.doi.org/10.1002/poc.2946 unless noted otherwise. There are numerous comments and details to these values. Please see the original articles for details and comments. GA values are given in kcal mol^-1. pK_a values in the gas phase are approximate and have been obtained from the GA values by dividing with 1.364 kcal mol^-1. ^b Ion pair pK_a values relative to picric acid in 1,2-dichloroethane. ^c Values from https://doi.org/10.1039/c7sc01424d. ^d Values from http://dx.doi.org/10.1021/jo101409p. ^e Value from E. V. Anslyn, D. A. Dougherty, Modern Physical Organic Chemistry, University Science Books, Sausalito, 2006. ^f Values from http://dx.doi.org/10.1021/jp506485x. DCE values have been recalculated from absolute to relative, in order to be comparable with the rest. ^g Values from http://dx.doi.org/10.1021/acs.jpca.6b02253. ^h Crude estimates from DCE data by considering that bulk water is by 53 kcal mol^-1 more basic than bulk DCE and bulk acetonitrile is by 42 kcal mol^-1 more basic than bulk DCE.