Oxidation pathways of hydroxydopamines and their chloride and bromide salts: an electrochemical study in acidic solutions

The anodic oxidation of 6-hydroxydopamine and 5-hydroxydopamine and their chloride and bromide salts was studied in aqueous acidic solutions. The electrodes used were Pt and Pt/M (UPD) (M = Bi, Tl, Pb) modified surfaces. The electrochemical studies revealed that the first two-electron oxidation leads to formation of reactive quinoid intermediates. Reactions of these quinoid intermediates with nucleophiles yield substituted quinones that are not stable in solution. The reactions are initiated by the oxidation of the third hydroxyl group at more positive potentials. Cyclic voltammetry and rotating ring-disc electrode voltammetry were used to elucidate the reaction mechanisms. These techniques provide powerful tools to generate and detect transient intermediates and products and give valuable information regarding the pathways and mechanisms of the redox reactions. Key words: 6-hydroxydopamine, 5-hydroxydopamine, voltammetry, electrolysis, oxidation mechanism, nucleophilic attack.