Activation of Methane and Ethane and Their Selective Oxidation to the Alcohols in Protic Media

The selective oxidation of methane and ethane to the alcohols in solvents ranging from strong acids to neutral aqueous medium has been studied. In 98% sulfuric acid, methane is oxidized to CH₃OSO₃H by a variety of 1e_- and 2e_- oxidants, such as S₂O₈^2-, Ce(IV), Pd(II), and Hg(II). In the case of ethane, the observed products are CH₃OSO₃H and HO₃SCH₂CH₂OSO₃H. These oxidations appear to proceed through a series of electron-transfer steps involving the intermediacy of radicals and carbocations. We have determined the ratio of rate constants for methane versus methanol oxidation by the Pt(II) ion in water and an average value of 0.17(2) was obtained. The similarity in rate constants for methane and methanol oxidations under such mild conditions is remarkable. Moving to substrates with C-H bonds somewhat weaker than that in methane results in actual reversalof commonly observed selectivity. As an example, we have observed the exclusiveoxidation of the methyl group in ethanol resulting in the formation of 1, 2-ethanediol as the predominant product. In addition, when ethane was included as a substrate, the relative rate of C-H bond activation by the Pt(II) ion decreasedin the order H-CH₂CH₃ > H-CH₂CH₂OH > H-CH(OH)CH₃. Another example of unusual selectivity was the observation of 1, 3-propanediol as the predominant product in the oxidation of 1-propanol by the Pt(II) ion in water.