Low-temperature oxidation of n-alkanes by cobaltic acetate activated by strong acids

The oxidizing activity of cobaltic acetate in acetic acid is enhanced by strong organic or inorganic acids to such an extent that n-alkanes can be significantly oxidized at low temperature. The kinetics of cobalt(III) consumption in the presence of n-heptane as substrate and trifluoroacetic acid as activator under a nitrogen atmosphere has been studied: the process appears to be of the second order with respect to cobalt(III) and it is markedly retarded by cobalt(II). The products of the oxidation of n-alkanes by acid-activated cobaltic acetate in acetic acid are different according to whether oxygen is present or not. Acetic esters of alcohols are formed under nitrogen while ketones are mainly produced if oxygen is present; when trichloroacetic acid is used as activator under nitrogen, alkyl chlorides are the principal products. In each case, the products consist of a mixture of position isomers with a marked predominance of the isomer resulting from attack of the alkane on the second carbon atom of the chain. These results are consistent with a mechanism wherein the n-alkanes are attacked by cobalt(III) to afford reversibly cobalt(II) and an alkyl free radical. The selectivities observed are discussed.