The reactions of peroxyl radicals occupy a central role in oxidative degradation. Under the term Advanced Oxidation Processes in drinking-water and wastewater processing, procedures are summarized that are based on the formation and high reactivity of the OH radical. These react with organic matter (DOC). With O2, the resulting carbon-centered radicals O2 give rise to the corresponding peroxyl radicals. This reaction is irreversible in most cases. An exception is hydroxycyclohexadienyl radicals which are formed from aromatic compounds, where reversibility is observed even at room temperature. Peroxyl radicals with strongly electron-donating substituents eliminate O2.−, those with an OH-group in a-position HO2.. Otherwise organic peroxyl radicals decay bimolecularly. The tetroxides formed in the first step are very short-lived intermediates and decay by various pathways, leading to molecular products (alcohols, ketones, esters and acids, depending on the precursor), or to oxyl radicals, which either fragment by scission of a neighbouring C-C bond or, when they carry an a-hydrogen, undergo a (water-assisted) 1,2-H-shift.

precursor: phosgene], formate (0.82) [precursor: formic peracid (0.88)], CO (0.04) [precursor: chloroformic peracid), dichloroacetic acid* (0.04). Some experiments were also done with tetramethylethene: acetone (1.7 4 ) [the precursor of half of the acetone, 2-hydroxypropyl-2hydroperoxide reacts too slowly with activated iodide to be quantified], 2,3-dimethyl-2,3-dihydroxybutane* (ca. 0.1). These results show that full mineralization is not achieved.

Formyl chloride has been generated in aqueous solution (i) by stopped-flow ozonation of vinyl chloride and (ii) by reacting dichloromethyl radicals with OH radicals using the pulse radiolysis technique. Vinyl chloride reacts in water with ozone (k = 1.7 × 104 dm3 mol-1 s-1, as determined by stopped-flow) yielding as final products (mol per mol of ozone) chloride ions (1.05), CO (1.01), and formate ions (0.06). Hydroxymethyl hydroperoxide (formaldehyde plus H2O2; 1.08) is also formed. HCl and formic acid are formed in less than 2 ms (the detection limit of the stopped-flow setup). At high pH the CO yield decreases (at pH 13.6 by 50%). It is concluded that the precursor of CO, HCl, and formic acid is formyl chloride. It predominantly decays into CO and HCl, and only at very high pH can hydrolysis to formic acid and HCl compete successfully. Using the pulse radiolysis technique dichloromethyl radicals are generated in Ar-saturated solutions from chloroform by reacting it with the solvated electron (originating from the radiolysis of water). The OH radicals (also from the radiolysis of water) partially react with the dichloromethyl radicals yielding dichloromethanol. Alternatively, dichloromethanol is generated in N2O-saturated solutions from dichloromethane, where some of the OH radicals are allowed to abstract an H atom from dichloromethane and another fraction to add to the dichloromethyl radicals. The observed conductivity changes are attributed to a very rapid decay (t 1/2 < 20 μs) of dichloromethanol into formyl chloride and HCl followed by the decay of formyl chloride into CO and HCl (k = 104 s-1). From these data and the decrease of the CO yield at high pH (ozonation of vinyl chloride) it is estimated that the OH--induced hydrolysis of formyl chloride occurs with a rate constant of ca. 2.5 × 104 dm3 mol-1 s-1.

Progressive hydrolytic decomposition of acyl chlorides, among them the chlorinated acetyl chlorides, which are produced in the gas-phase oxidation of chlorinated ethylenes, permits the complete mineralization of organically bound chlorine to chloride anion. Hydrolysis rate constants (100% water) have been determined for the following acyl chlorides: acetyl (350 s Ϫ1 ), chloroacetyl (5.5 s Ϫ1 ), dichloroacetyl (300 s Ϫ1 ), trichloroacetyl (>350 s Ϫ1 ), and oxalyl dichloride (>350 s Ϫ1 ). The chlorinated acetyl chlorides thereby give rise to the chloroacetates whose decomposition has also been studied and the kinetic parameters determined. Mono-and dichloroacetate anion undergo hydrolytic dechlorination (). Trichloroacetate anion decomposes by another mechanism, undergoing decarboxylation which is base-uncatalyzed: A o 2.1 × 10 17 s Ϫ1 , E o 146 kJ mol Ϫ1 . Procedures on a pilot-plant scale are pointed out that allow the elimination of these compounds upon oxidation of the strip-gas produced when contaminated water is freed from chlorinated ethylenes by air-stripping.