The polarography and coulometry of the electro-oxidation of monohalogenated 9,10-dihydroxyanthracenes

Abstract

The polarographic oxidation at the dropping mercury electrode of all eight of the monohalogenated 9,10-dihydroxyanthracenes and of the sodium 2-chloro-6-sulphonate and 2-bromo-5-sulphonate in 0·1 N-NaOH in 50% EtOH–H₂O is reversible, and involves two electrons per molecule. In aqueous 0·1 N-NaOH, the polarograms are distorted by adsorption effects which are quantitatively explained by assuming partial blocking of the electrode surface by insoluble quinone. The semiquinone formation constants are considerably larger in the 50% alcoholic solvent than in water. Controlled-potential macrocoulometry shows that during electroreduction and reoxidation two electrons per molecule are transferred for all the anthraquinones except the 1-iodo-derivative, the anomalous results for which are due to partial dehalogenation, during electrolytic reduction and oxidation. For two halogenated anthraquinonoid vat dyes, Caledon Brilliant Orange 6R and Cibanone Golden Yellow RK, previously thought to undergo 1-electron electro-oxidation of the leuco-form with accompanying partial dehalogenation, coulometry shows that both reduction and reoxidation are two-electron processes, and previous discrepant results may be explained by absorption or complexation of leuco-dye by quinone, or alternatively by strong adsorption of quinone on the dropping mercury electrode.