Ferrate(VI) Oxidation of Aqueous Phenol:  Kinetics and Mechanism

Abstract

Kinetic and thermodynamic parameters for ferrate(VI) oxidation of phenol have been measured in isotopic solvents, H2O and D2O, using ambient and high-pressure stopped-flow UV−visible spectroscopy. An increase (fast stage) and then a decrease (slow stage) in absorbance at 400 nm are observed when potassium ferrate (K2FeO4) and aqueous phenol solutions are mixed rapidly. This suggests that small amounts of unstable intermediate 4,4'-biphenoquinone are produced during this redox process. An electron paramagnetic resonance signal for the reaction mixture of ferrate and phenol trapped by spin-trap α-(4-pyridyl-1-oxide)-N-tert-butylnitrone indicates a radical reaction pathway. Gas chromatographic/mass spectrometric measurements show p-benzoquinone is a major organic product, and the red ferric thiocyanate complex formed from addition of potassium thiocyanate to the spent reaction solution indicates that Fe(VI) is reduced to Fe(III). Activation enthalpy, entropy, and volume changes have been determined. There is a primary isotope effect for the formation of the intermediate (fast stage), kfast(H2O)/kfast(D2O) = 2.4 ± 0.6. Because the phenol hydroxylic hydrogen is deuterated in D2O, this isotope effect suggests that a hydrogen bond is formed in the transition state.