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, λ_max(F₅C₆O˙) = 395 nm; k(N₃˙ + Cl₅C₆O⁻) = 5.7 × 10⁹ dm³ mol⁻¹ s⁻¹, λ_max(Cl₅C₆O˙) = 452 nm; k(N₃˙ + Br₅C₆O⁻) = 6.5 × 10⁹ dm³ mol⁻¹ s⁻¹, λ_max(Br₅C₆O˙) = 476 nm; k(N₃˙ + I₃H₂C₆O⁻) = 5.6 × 10⁹ dm³ mol⁻¹ s⁻¹, λ_max(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 (t_1/2 k 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.