Electro-oxidation of alkylnaphthalenes

Naphthalene, mono-methyl-, mono-ethyl-, and several dimethyl-naphthalenes have been electro-oxidized at a Pt electrode in 1:1 acetone + water solution, containing the supporting electrolyte 0.1 M Na2SO4 and a redox mediator, 0.02 M CoSO4. The electric charge passed corresponded to ca. 4–7 electrons/molecule relative to the amount of substrate in the electrolysis (ca. 0.5–1 mmol). The products were analyzed by GC-MS and NMR, and separated by column chromatography on silica gel. Alkyl-substituted naphthoquinones and binaphthyls have been found to be the main products. The mechanism of the electro-oxidation is discussed, in which unstable radical cations are formed, initially, at the electrode or in solution, in a reaction with the mediator Co+3 ions.