Electrochemical reduction of parabanic acid: Products and mechanism

Abstract

The polarographic reduction of parabanic acid, methylparabanic acid and dimethylparabanic acid has been investigated over a wide range of pH. The primary electrochemical reaction involves overall two-electrons and two-protons to give the corresponding 5-hydroxyhydantoin. At low pH and possibly at moderately high pH, the rate controlling step involves two-electrons and two-protons; at intermediate pH values a two-electron, one-proton rate controlling process is operative. 5-Hydroxyhydantoin appears to be slowly hydrolyzed to glyoxylic acid and urea so that prolonged electrolysis (in 1 M HOAc) of parabanic acid gives a mixture of 5-hydroxyhydantoin, urea and the reduction product of glyoxylic acid, namely glycolic acid. In phosphate-containing buffers between approximately pH 6 and 8 all the parabanic acids give a very weak complex with phosphate which is reduced at more negative potentials than the free acids in an essentially pH independent process. On the basis of the phosphate species present in solution and the pH independence of the complex wave, a tentative structure has been proposed for the complex.