Strong inhibition of the Fe³⁺ + H₂O₂ reaction by ethanol: Evidence against the free radical theory

The effect of ethanol on the catalytic decomposition of H₂O₂ by Fe³⁺ was investigated. While expecting a simple competitive oxidation of C₂H₅OH, far more complex kinetics were encountered experimentally: already minute amounts of C₂H₅OH (1% of H₂O₂) had a powerful retardation effect on the disappearance of H₂O₂. This fact indicated the operation of an intricate mechanism. It excluded the possibility of OH· radicals being the active agents in the oxidation: OH· radicals generated by radiolysis react with C₂H₅OH with a very high rate constant. The interpretation of the experimental results was based on a mechanism involving iron in a +5 oxidation state (FeO³⁺) as the active intermediate and its binding in complex structures in which activity is reduced. The question of free radical versus non-radical mechanisms is discussed. The conclusions differ from generally accepted concepts in relation to the Fenton and related reactions.