Photo‐induced disproportionation of iodomalonic acid

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°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 100% conversion. At pH < 2.9, I2 and malonic acid are also formed and the reaction stops at a conversion rate less than 100%. 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: equation image equation image 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.