Kinetics and mechanisms of the oxidation of methyl aryl ketones by acid permanganate

Abstract

The oxidation of six methyl aryl ketones by acid permanganate has been studied, in the presence of fluoride ions. The products of the oxidation are formaldehyde and the corresponding benzoic acids.The oxidation is first order with respect to each the ketone, the oxidant and hydrogen ions. The reaction rate increases sharply with the increase in the amount of acetic acid in the solvent. The oxidation of acetophenone exhibits the kinetic isotope effect, k_H/k_D = 4.60 at 30°C. The solvent isotope effect is given by k(D₂O)/k(H₂O) = 5.03. The acid-catalysed enolization rate of the ketones has been measured by the bromination method. The activation parameters for both the oxidationand enolization reactions have been evaluated. The oxidation is slower than the acid-catalysed enolization. The relative rates of the oxidation are proportional to the relative rates of enolization.These results coupled with the magnitude of the solvent isotope effect suggest that the enol form of the ketone is involved in the oxidation process. A possible mechanism has been suggested.