Evaluation of acidity of strong acid catalysts I. The strength of boron trifluoride-water systems

Abstract

The method of determining a thermodynamic acidity function from the chemical shift changes of 13C signals of unsaturated ketones at infinite dilution in the investigated acid established by the authors was applied to the system boron triftuoride-water ( III) ranging from the monohydrate (BF 3 · H 2O) to the trihydrate (BF 3 · 3H 2O). The indicators used were mesityl oxide and 4-hexen-3-one. It was found that III is significantly stronger than indicated by earlier measurements conducted by the classical Hammett method based on UV-visible spectroscopy. The mixtures with about 1.25 mol of water per mol of BF 3 or less are stronger than pure sulfuric acid and are therefore superacidic. The stronger acidity of III can be understood because boron triftuoride is a much stronger Lewis acid than sulfur trioxide; therefore the complex with a hydroxyl anion of the former (hydroxytrifluoroborate anion) should have a lower affinity for a hydron than the corresponding complex of sulfuric anhydride (bisulfate anion). Preliminary experiments indicate that the 13C NMR method can be applied successfully to working catalysts based on III, which are colored and contain dissolved organic materials.