The Hammett Acidity Function for Hydrofluoric Acid and some related Superacid Systems

Abstract

<p>The Hammett acidity function, H₀, has been measured for some superacid systems. These systems include HSO₃F, CF₃SO₃H and HF and their mixtures with some Lewis acids such as NbF₅, TaF₅, AsF₅ and SbF₅. Some base-HF mixtures were also studied for the HF system.</p> <p>The acidities of these systems were determined by measuring their ability to protonate the very weakly basic nitro aromatic indicators. The protonation behaviour was studied by means of UV-Visible spectrophotometry as well as NMR spectrometry.</p> <p>The H₀ value for 100% HF was found to be greater than that of 100% H₂SO₄ and was close to that of 100% HSO₃F. The acidities for the acid systems MF₅HF were found to be in the order SbF₅> AsF₅> TaF₅> NbF₅. The H₀ values obtained in the present measurements indicated that the SbF₅HF system is much more acidic than the previously known most highly acidic system, HSO₃F-SbF₅-3SO₃.</p> <p>The applications of the superacid media in synthetic chemistry has become more important in recent years, for example they have been used as solvents in the presentation of carbonium ions and other highly electrophilic cations.</p>