The electrochemistry of aqueous solutions of 4-methoxy-1,2-benzoquinone

Abstract

Using cyclic voltammetry, the electrochemistry of aqueous solutions of 4-methoxy-1,2-benzoquinone was investigated over the pH range 0–8. From the variation of half-wave potential with pH, it was inferred that the quinone was reduced to the hydroquinone; no acid—base reactions involving either the quinone or hydroquinone were observed. At low pH (<3), the cyclic voltammogram of the benzoquinone changed rapidly to a second pair of peaks at lower potentials. These were shown to be due to 2-hydroxy-1,4-benzoquinone, formed by hydrolysis of the original compound. The other hydrolysis product, methanol, was detected chromatographically. The hydrolysis reaction was shown to be first order both with respect to the 1,2-benzoquinone and protons. At pH 6 and above, 4-methoxy-1,2-benzoquinone showed a second type of decomposition whose rate was approximately first order in hydroxide ion. Using electrochemical and spectroscopic measurements, it was also demonstrated that 4-methoxy-1,2-benzoquinone was formed during the tyrosinase-catalysed oxidation of 4-methoxyphenol. This confirms that this quinone could be involved in the melanocytoxic action of 4-methoxyphenol.