Aqueous trifluoroacetic acid as a medium for organic reactions. I. Acidity functions and the identity of the manganese(VII) species found in powerfully acidic media

Abstract

The HR acidity function has been determined for the trifluoroacetic acid–water system, and extended beyond 100% acid by the addition of trifluoroacetic anhydride, to 99.5 wt.% anhydride. The existing H0 acidity function is corrected and extended to 100% trifluoroacetic acid, confirming Eaborn's observation that H0 attains a minimum value at approximately 97% acid. Comparison of the acidity functions shows that although trifluoroacetic acid is only weakly protonating, it is a strongly protonating-dehydrating medium. Changes in the uv–visible spectrum of potassium permanganate produced by changing acid concentration are used to calculate the degree of further ionization of the permanganate ion; the correlation of this with the HR acidity function further suggests the presence of the permanganyl cation, MnO3+, or its trifluoroacetate complex or ester, MnO3•O2CCF3, in these solutions. A cryoscopic investigation into the mode of ionization of permanganate in trifluoroacetic acid reveals that extensive ion-pair formation occurs in this solvent. Although this effect prevents an unequivocal determination of the number of solute particles being made the results are consistent with the presence of a permanganyl species in solution.