Electrochemical oxidation of barbituric acids in aqueous solution in the absence of chloride ion

Abstract

Barbituric acid, 1-methylbarbituric acid, 1,3-dimethylbarbituric acid and 1,3-diethylbarbituric acid are electrochemically oxidized at the pyrolytic graphite electrode by way of two principal voltammetric oxidation processes. The reaction schemes and products formed in these processes were studied in most detail for 1,3-dimethylbarbituric acid. The first voltammetric peak I a involves theoretically 1.33 e per molecule of 1,3-dimethylbarbituric acid oxidized. The reaction scheme involves an initial 1 e oxidation of 1,3-dimethylbarbituric acid to give a neutral radical that dimerizes at the 5-position to give hydurilic acid. This is then immediately further electrooxidized in a 2 e/1H + process to give a carbonium ion that reacts further with barbituric acid to give 5,5-di[1′,3′-dimethyl-2′,4′,6′-trioxopyrimid(5)yl]-1,3-dimethyl-2,4,6-trioxopyrimidine, the so-called open chain trimer of 1,3-dimethylbarbituric acid. The second voltammetric oxidation peak II a is rather more complex. Electrochemical oxidation of the open chain trimer of 1,3-dimethylbarbituric acid at peak II a potentials involves a 2 e/2H + reaction to give 5,6-dihydro-1,3-dimethyl-5,6-di[1′,3′-dimethyl-2′,4′,6′-trioxopyrimid-(5′,5′)yl]furo[2,3-d]uracil, the so-called cyclic trimer of 1,3-dimethylbarbituric acid. Direct electrooxidation of 1,3-dimethylbarbituric acid at peak II a potentials however, involves two reaction routes. The first via an initial 1 e process to a radical that dimerizes and ultimately forms open chain trimer as in the peak I a process, however the open chain trimer is then further electrooxidized to the cyclic trimer. The second route appears to involve a further 1 e oxidation of the initially formed barbituric acid radical to give a carbonium ion. This is rapidly attacked by water to give dialuric acid which in turn is immediately further oxidized to 1,3-dimethylalloxan. Barbituric acid and other N-substituted barbituric acids appear to be electrochemically oxidized by similar mechanisms.