Equilibrium protonation of the 1,3,5-trinitrobenzene–phenoxide σ-complex

Abstract

The protonation behavior of the trinitrobenzene–phenoxide σ-complex has been determined in aqueous sulfuric acid, aqueous hydrochloric acid, and ethanolic hydrogen chloride. The conjugate acid of the σ-complex, a nitronic acid, exhibits acidic properties similar to those of picric acid and the cyanocarbon acids. The acidity (pK a ) of the nitronic acid has been evaluated with the aid of various acidity function techniques and using the Bunnett–Olsen method. The nitronic acid is ca. 1 pK unit stronger than picric acid. However, both the nitronic acid and picric acid are considerably weaker acids in ethanolic than in aqueous media (ΔpK ca. 4 units). The results provide a preliminary H − acidity function scale for aqueous and ethanolic hydrogen chloride.