Oxidation of Hydroxymalonic Acid by OH Radicals in the Presence and in the Absence of Molecular Oxygen. A Pulse-Radiolysis and Product Study

Article abstract of DOI:10.1021/j100022a026

The rate constants of the reaction of OH radical/O^- radical with hydroxymalonic acid (1) have been determined at various pH, the OH radical being generated pulse-radiolytically.This reaction involves the abstraction of an H atom at C(2), i.e., the formation of the α-hydroxyalkyl radical (2), and is fastest in the pH range from 6 to 10 (k = 4.4E8 dm³ mol^-1 s^-1) where hydroxymalonic acid exists as the dianion.The pK_a values of the carboxyl functions of radical 2 have been found to be 1.9 and 4.8, similar to, or slightly higher than, those of the parent compound (1.89 and 4.28 at 25 deg C).However, the pK_a of the alcohol function of radical 2 (found to be 12.9) is several units lower than that of the parent compound.In the presence of oxygen radical 2 is rapidly converted into the corresponding peroxyl radical 3 (k = 1.6E9 dm³ mol^-1 s^-1).The α-hydroxyperoxy radical (3) eliminates HO2 radical with a rate constant of 1.1E4 s^-1 to give ketomalonic acid.In the alkoxide form, peroxyl radical 3 eliminates O2^- radical faster (8.9E4 s^-1).The pK_a of the alcohol function in 3 has been estimated at 9.8, much lower than that of radical 2.In the γ-radiolysis of hydroxymalonic acid in N2O/O2-saturated solutions, product ratios change with pH.At pH 3, the main products (4:1 v/v) are ketomalonic acid (G = 5.1E-7 mol J^-1) and hydrogen peroxide.At pH 10 oxalic monoperacid and carbon dioxide become major products.These products are formed in a (slow) chain reaction involving ketomalonic acid and the superoxide radical.Ketomalonic acid also reacts with the product hydrogen peroxide (k = 5E-3 dm³ mol^-1 s^-1), yielding peroxides which, depending on pH, can be present in different states of protonation.