Abstract
A detailed picture is given of the mechanisms involved in the oxidation of acetophenones with oxygen to benzoic acids, at 150°, using manganese salts as catalysts and butyric acid as solvent.
Kinetic studies indicate that the reaction is first‐order with respect to ketone and butyric acid, except at very high acid concentrations, and independent of the manganese concentrations as long as these exceed 0.008 M.
The results are most satisfactorily described by a mechanism involving enolization of the ketone as the rate‐determining step. The enol is then assumed to be oxidized by Mn^+++^ to form the phenacyl radical, which adds on oxygen to form the phenacyl peroxy radical. This in turn readily reoxidizes Mn^++^ to Mn^+++^; the phenacyl peroxy anion rearranges to form formaldehyde and the benzoate anion.
Supporting evidence for this postulated scheme has been obtained from a study of some electron transfer reactions of side‐chain‐substituted acetophenones and of other compounds with manganic ions.
When cobalt ions instead of manganese ions are used as the catalyst, the reaction becomes much slower and is first order in cobalt ion. The cause to which this difference is attributed is that now the re‐oxidation of cobaltous ion by the phenacyl peroxy radicals becomes rate‐determining, the oxidation potential of the cobalt ions being appreciably higher than that of the manganese ions.
Electron‐withdrawing nuclear substituents were found to have a retarding effect, whereas electron‐supplying substituents had an accelerating effect, probably on account of their influence on enol formation.