The reaction of the OH radical with pentafluoro-, pentachloro-, pentabromo- and 2,4,6-triiodophenol in water: electron transfer vs. addition to the ring

Abstract

The OH-radical-induced dehalogenation of pentafluorophenol (F5C6OH), pentachlorophenol (Cl5C6OH), pentabromophenol (Br5C6OH) and 2,4,6-triiodophenol (I3H2C6OH) in water has been studied by pulse radiolysis in basic solution where these compounds are deprotonated and hence slightly water soluble. Hydroxyl radicals react with these phenolates both by electron transfer and by addition. Electron transfer yields hydroxide ions and the corresponding phenoxyl radicals (X5C6O˙ and I3H2C6O˙); these were also generated independently, to the exclusion of OH-adduct radicals, by reacting the phenolates with N3 radicals [k(N3˙ + F5C6O−) = 4.9 × 109 dm3 mol−1 s−1, λmax(F5C6O˙) = 395 nm; k(N3˙ + Cl5C6O−) = 5.7 × 109 dm3 mol−1 s−1, λmax(Cl5C6O˙) = 452 nm; k(N3˙ + Br5C6O−) = 6.5 × 109 dm3 mol−1 s−1, λmax(Br5C6O˙) = 476 nm; k(N3˙ + I3H2C6O−) = 5.6 × 109 dm3 mol−1 s−1, λmax(I3H2C6O˙) = 540 nm]. Hydroxyl radical addition to the pentahalophenolates is followed by rapid halide elimination, giving rise to hydroxytetrahalophenoxyl radical anions (X4O−C6O˙). The latter exhibit absorption maxima near those of the pentahalophenoxyl radicals. This prevents a proper determination of the relative importance of the two processes by optical detection. However, these two processes distinguish themselves by their behaviour with respect to the stoichiometry and kinetics of the production of ionic conducting species. In basic solution, electron transfer causes a conductivity increase due to the formation of OH− whereas addition followed by HX elimination and deprotonation of the X4OHC6O˙ radical results in a conductivity drop. The evaluation of the conductivity change at 8 μs after the radiolytic pulse has ended, reveals that about 27%, 53%, 73%, and 97% of the OH radicals react by electron transfer with F5C6O−, Cl5C6O−, Br5C6O− and I3H2C6O−, respectively. Further conductivity change occurs during the bimolecular termination of the halophenol-derived radicals (t1/2 <1 ms, 2k range between 1.2 × 109 and 4 × 109 dm3 mol−1 s−1) and continues into progressively longer times, owing to the hydrolysis of unstable HX-releasing products, on account of the replacement of OH− by halide/halophenolate ions. Additionally, further halide is released on a time scale of minutes and hours. The rates of the conductivity change in the time range from a few ms to several tens of seconds are proportional to the OH− concentration.