Solutes in sulfuric acid. Part VIII. Protonation of phenol, 4‐fluorophenol and the 2‐ and 4‐sulfonates of anisole and phenol in concentrated aqueous sulfuric acid; determination of pKa values by means of 13C NMR

Abstract

AbstractThe protonation of phenol (2), 4‐fluorophenol (3), phenyl methanesulfonate, potassium phenyl sulfate and the potassium 2‐ and ‐4‐sulfonates of both phenol and anisole in concentrated aqueous sulfuric acid at 30°C has been investigated using 13C NMR spectroscopy. The results show that phenol and 4‐fluorophenol are protonated predominantly on the oxygen atom. This protonation is governed by the H0a acidity function. The pKa of the oxonium ions of 2 and 3 are ‐6.40 ± 0.05 and ‐6.41 ± 0.05, respectively. The protonation of phenyl methanesulfonate occurs at acid concentrations > 100%. Potassium phenyl sulfate in ≥20% H2SO4 hydrolyses very rapidly to give phenol. The 2‐ and 4‐sulfonates of both phenol and anisole are protonated on both the ‐OR (R = H, Me) and the ‐SO3− groups; the sulfuric acid concentrations of half‐protonation have been determined to be between 80.0 and 83.3% H2SO4, corresponding with pKa's of the conjugate acids between ‐6.34 and ‐6.66 based on the H0a scale.