Photo-Induced Disproportionation of Iodomalonic Acid

Article abstract of DOI:10.1002/kin.550270503

The stoichiometry, equilibrium, and kinetics of the photo-induced disproportionation of iodomalonic acid to I-, I2, and tartronic acid have been studied by means of spectrophotometry and iodide selective electrode at 20.0 +/- 0.2 deg C, pH 2.0 - 4.0.At pH > 2.9, only I- and HOCH(COOH)2 are detected as major products and the reaction reaches 100percent conversion.At pH < 2.9, I2 and malonic acid are also formed and the reaction stops at a conversion rate less than 100percent.Both UV (band with a peak at 360 nm) and visible light (480 nm) have been found to be effective.Two primary photochemical processes are identified: (M1) ICH(COOH)2 + hν <-> I^. + ^.CH(COOH)2, (M2) I2 + hν <-> 2I^..While both reactions are sensitive to UV light, only M2 can be affected by visible light. (M1) and (M2) are considered to initiate a chain reaction sequence in which I^. radicals oxidize iodomalonic acid.Dual effects of reaction products on the reaction rate have been observed: while iodine increases the efficiency of visible light and accelerates the reaction, malonic acid inhibits the photo-decomposition by mediating the recombination of I^. radicals to I2.