The cobaltic acetate oxidation of alkyl aromatic hydrocarbons was studied and an electron-transfer mechanism involving the intermediacy of aromatic radical cations was proposed. This mechanism was indicated by the high p value of -2.4 (correlated with + at 65°) obtained from the relative reactivity of substituted toluenes, and the unusually low reactivity of cumene relative to toluene. The radical cation intermediates were detected by epr flow techniques in a few cases. The addition of lithium chloride lead to a dramatic enhancement of the reactivity of cobaltic acetate. In the presence of chloride ions the relative reactivity of substituted toluenes yielded a P value of -1.35 (correlated with + at 24°). The effect of chloride ions on the p value was attributed to the formation of a cobaltic complex of relatively high oxidation potential and lower selectivity. The effect of chloride ion concentrations on the nuclear vs. side-chain products was also studied.Although the cobalt acetate catalyzed air oxidation of l alkylbenzenes to the corresponding mono-and dicarboxylic acids forms the basis of several commercial processes,2 the mechanistic role of cobalt is not very clear. The requirement of relatively high cobalt con-
The manganic acetate oxidation of aromatic hydrocarbons proceeds by two competing mechanisms: (a) a free-radical mechanism resulting from the interaction of the aromatic hydrocarbon with the carboxymethyl radical (• CH2COOH) generated directly by the thermolysis of manganic acetate ; (b) an electron transfer mechanism which is of importance in the oxidation of aromatic hydrocarbons having ionization potentials of <8 eV. The relative extent of the electron transfer pathway can be suppressed by carrying out the reaction under anhydrous conditions or in the presence of potassium acetate. The relative reactivity of the carboxymethyl radical toward hydrogen abstraction from substituted toluenes was determined yielding a p value of -0.63 (correlated with +). The observed relative rate ratio of hydrogen atom abstraction from toluene-diphenylmethane-triphenylmethane is 1:12:39, respectively. A study of the relative rates of addition of the CH2COOH radical to substituted benzenes revealed that the reactivity toward addition is guided by the stability of the radical intermediate irrespective of whether the groups are electron withdrawing or electron donating.
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