Strength of liquid acids in solution and on solid supports. The anion stabilization by solvent and its consequences for catalysis

The effect of solvents upon the effective strength of acids in solution was studied in the strong acid range by the measurement of the Δ δ parameter for mesityl oxide at stoichiometric acid/base ratio (Δ δ1) and in the weak superacid range by the measurement of the hydronation of hexamethylbenzene (HMB). The approach is applicable to acids which cannot be described by an acidity function (non-Hammett acids). For sulfuric acid, the strength given by the Δ δ1 parameter changes with solvent in the order: sulfolane < neat acid < SO2 < hexafluoroisopropanol (HFIP). Thus, sulfolane behaves like a basic solvent toward H2SO4. The small effect of SO2 is a result of its being a polar solvent. The large acidity enhancement observed in HFIP solution results from its ability to form strong hydrogen-bonded complexes with the acid anion (anion stabilization). The extent of hydronation of HMB by trifluoromethanesulfonic acid (TFMSA) changes with solvent in the order: SO2, SO2ClF < trifluoroacetic acid (TFA) < HFIP. As TFA is more basic than SO2, this finding demonstrates that TFA and HFIP are particularly good anion stabilizing solvents. Basicity of a solvent is not well described by the pKBH+ measured for that compound as a solute, but can be assessed from the decrease in the extent of hydronation of a probe base by an acid in the solvent. For hydrogen-bond donor solvents, a correction has to be made for the anion stabilizing effect (acidity-enhancing). An empirical relative solvent basicity parameter (SB) was developed from the examination of hydronation of HMB by TFMSA (3 mol) in TFA-CHCl3 solutions, and its suppression by the addition of an amount of solvent i equal to the acid (SB(i, TFA)). TFA is thus taken as the standard, non-basic, solvent, and also provides the anion stabilization. Values of SB(i, TFA) for some carboxylic acids and one nitroalkane are listed. The effect of anion stabilizing solvents as promoters for strong acid catalysts was shown by the acceleration of the transalkylation of p-di-tert-butylbenzene (p-DTBB) with toluene catalyzed by H2SO4, upon addition of small amounts of TFA or HFIP (most effective) to the acid.