Intermediate steps in autoxidations: II. Autoxidation and enolization of acetophenone in acetic acid

Abstract

The manganic acetate-catalyzed autoxidation (110 °C, 1 atm of oxygen) of acetophenone (<0.7 M) in CH 3COOD as well as of acetophenone-ω- d 3 in CH 3COOH proceed at the same rates as the H D exchange under the same conditions but in the absence of oxygen. In both oxidation reactions, the unoxidized ketones retain their isotopic compositions. Moreover, substituent effects on autoxidations of acetophenones are the same as observed for H D exchange (ϱ ≈ −0.7). These data constitute definite evidence for enolization being the rate-determining step in these autoxidations. Arrhenius energies of activation are of the order of 15 kcal for autoxidation as well as for deuteration of acetophenone. At higher concentrations, however, autoxidation rates fall below rates of H D exchange; the unoxidized acetophenones show isotopic exchange. In these systems, reduction of trivalent manganese by the enol is probably so fast that its regeneration from Mn II becomes (partly) rate-determining. H D isotope effects are small when changing the solvent from CH 3COOH to CH 3COOD, rates decreasing by a factor of 1.2 at 110 °C. However, C 6H 5COCD 3 is oxidized in acetic acid at a three times lower rate than acetophenone.