Electrochemical oxidation of p-methoxytoluene in acetic acid saturated by molecular oxygen

The direct electrochemical oxidation of p-methoxytoluene (MT) in solution in acetic acid on a graphite anode under oxygen-free conditions results in the formation of (MBA) with chemical yields higher than 80%. This paper shows that the same process, operated under oxygen saturation conditions, concurrently leads to the formation of p-methoxybenzaldehyde. The acetate/aldehyde ratio varies from 37 under an oxygen-free atmosphere to 2.3 under oxygen saturation (1 atm, 80°C, 125 A m−2). The results of voltammetric studies on the one hand, and those of large-scale electrolyses of MT and MBA on the other hand, permit us to propose a mechanism for the process conducted under oxygen saturation. Accordingly, it is assumed that the benzyl radicals generated on the graphite anode, from MT or MBA in acetic acid, can react directly with the dissolved molecular oxygen. The generated peroxide radicals, ArCH2O2, undergo chemical reactions leading to the formation of the corresponding aldehyde and alcohol. Under the reaction conditions, most part of the alcohol is electro-oxidized into aldehyde. As the process is run in the absence of complexes of transition metals, the aldehyde obtained is protected from further oxidation.