Formylmethyl and Formylmethylperoxyl Radicals and Their Chlorinated Derivatives. Formation and Reactions in Irradiated Aqueous Solutions

The formylmethyl radical, ^̇CH₂CHO, is known to act as an oxidant. For comparison, the chlorinated analogues, ^̇CCl₂CHO and ^̇CCl₂CClO, were produced in irradiated aqueous solutions and their oxidation reactions studied. The radicals were produced by reaction of ^̇OH with trichloroethanol (CCl₃CH₂OH) or tetrachloroethylene (CCl₂=CCl₂) followed by elimination of HCl. CCl₃CH₂OH reacts with OH radicals, with rate constants of 1.5 × 10⁸ L mol^-1 s^-1 at pH 6.6 and 1.9 × 10⁹ L mol^-1 s^-1 at pH 11.4, at which the compound is partially ionized (pK_a = 12.2). These reactions take place by hydrogen abstraction to yield CCl₃CHOH or CCl₃CHO^-, which rapidly eliminate HCl or Cl^- to form the ^̇CCl₂CHO radical. Parallel experiments with trifluoroethanol indicated formation of the CF₃CHOH radical which does not eliminate HF. This radical reduces methylviologen (MV²⁺, 1,1′-dimethyl-4,4′-bipyridinium dication) slowly in acidic solutions but very rapidly in alkaline solutions. From the pH effect on the rate constant, a pK_a value of 6.0 was determined for the acid dissociation of CF₃CHOH into CF₃CHO^-. The ^̇CCl₂CHO radical is an oxidizing species, which reacts with ascorbate, ABTS^2- [2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate ion)], and phenolate ions somewhat more slowly than *CH₂CHO. In contrast, *CCl₂CClO reacts with the same compounds much more rapidly than *CH₂CHO. All three formylmethyl radicals react with O₂ to yield peroxyl radicals. The peroxyl radicals derived from the chlorinated formylmethyl are stronger oxidants than their parent species, but the peroxyl radical derived from the unchlorinated formylmethyl is a weaker oxidant than its parent species. Peroxyl radicals were also produced by the reaction of e_aq^- with CCl₃CH₂OH and with chloral hydrate in aerated solutions. These radicals, ^̇OOCCl₂CH₂OH and ^̇OOCCl₂CH(OH)₂, are highly reactive oxidants.