Electrochemical oxidation of barbituric acids at low pH in the presence of chloride ion

Barbituric acid, 1-methylbarbituric acid and 1,3-dimethylbarbituric acid are electrochemically oxidized at the pyrolytic graphite electrode by way of a single voltammetric peak at pH 1 in the presence of chloride ion. At least four products are formed as a result of the reaction, the three major products, accounting for more than 80–90% of the oxidized barbituric acid, are the appropriately N-methylated 5,5′-dichlorohydurilic acids, 5,5-dichlorobarbituric acids and alloxans. The mechanism appears to proceed by an initial potential-controlling 1e/1H⁺ oxidation of the barbituric acids to give a barbituric acid radical. This can dimerize to hydurilic acid, which is then further electrochemically oxidized. However, this appears to be a minor route. The barbituric acid radical appears to be mainly further electrooxidized (1e) to a carbonium ion which further reacts with nucleophiles such as chloride ion to give 5-chlorobarbituric acid, or with water to give dialuric acid. Further electrochemical oxidation and chemical reactions of the latter species results in formation of the ultimate products.