PHOTOINDUCED DISPROPORTIONATION OF IODOMALONIC ACID

The stoichiometry, equilibrium, and kinetics of the photo-induced disp roportionation of iodomalonic acid to I-, I-2, and tartronic acid have been studied by means of spectrophotometry and iodide selective elect rode at 20.0 +/- 0.2 degrees C, pH 2.0-4.0. At pH > 2.9, only I- and H OCH(COOH)(2) are detected as major products and the reaction reaches 1 00% conversion. At pH < 2.9, I-2 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 b e effective. Two primary photochemical processes are identified:(M1) I CH(COOH)2 + h upsilon reversible arrow I-. + (CH)-C-.(COOH)(2) (M2) I- 2 + h upsilon reversible arrow 2I(.) While both reactions are sensitiv e to UV light, only (M2) can be affected by visible light. (M1) and (M 2) are considered to initiate a chain reaction sequence in which I-. r adicals oxidize iodomalonic acid. Dual effects of reaction products on the reaction rate have been observed: while iodine increases the effi ciency of visible light and accelerates the reaction, malonic acid inh ibits the photo-decomposition by mediating the recombination of I-. ra dicals to I-2. (C) 1995 John Wiley & Sons, Inc.