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

The OH-radical-induced dehalogenation of pentafluorophenol (F5C6OH), pentac hlorophenol (Cl5C6OH), pentabromophenol (Br5C6OH) and 2,4,6-triiodophenol ( I3H2C6OH) in water has been studied by pulse radiolysis in basic solution w here these compounds are deprotonated and hence slightly water soluble. Hyd roxyl radicals react with these phenolates both by electron transfer and by addition. Electron transfer yields hydroxide ions and the corresponding ph enoxyl radicals (X5C6O. and I3H2C6O.); these were also generated independen tly, to the exclusion of OH-adduct radicals, by reacting the phenolates wit h N-3 radicals [k(N-3(.)+F5C6O-)=4.9x10(9) dm(3) mol(-1) s(-1), lambda(max) (F5C6O.)=395 nm; k(N-3(.)+Cl5C6O-)=5.7x10(9) dm(3) mol(-1) s(-1), lambda(ma x)(Cl5C6O.)=452 nm; k(N-3(.)+Br5C6O-)=6.5x10(9) dm(3) mol(-1) s(-1), lambda (max)(Br5C6O.)=476 nm; k(N-3(.)+I3H2C6O-)=5.6x10(9) dm(3) mol(-1) s(-1), la mbda(max)(I3H2C6O.)=540 nm]. Hydroxyl radical addition to the pentahalophen olates is followed by rapid halide elimination, giving rise to hydroxytetra halophenoxyl radical anions ((X4O-C6O.)). The latter exhibit absorption max ima near those of the pentahalophenoxyl radicals. This prevents a proper de termination of the relative importance of the two processes by optical dete ction. However, these two processes distinguish themselves by their behavio ur with respect to the stoichiometry and kinetics of the production of ioni c conducting species. In basic solution, electron transfer causes a conduct ivity increase due to the formation of OH- whereas addition followed by HX elimination and deprotonation of the X4OHC6O. radical results in a conducti vity drop. The evaluation of the conductivity change at 8 mu s after the ra diolytic pulse has ended, reveals that about 27%, 53%, 73%, and 97% of the OH radicals react by electron transfer with F5C6O-, Cl5C6O-, Br5C6O- and I3 H2C6O-, respectively. Further conductivity change occurs during the bimolec ular termination of the halophenol-derived radicals (t(1/2) < 1 ms, 2k rang e between 1.2x10(9) and 4x10(9) dm(3) mol(-1) s(-1)) and continues into pro gressively longer times, owing to the hydrolysis of unstable HX-releasing p roducts, on account of the replacement of OH- by halide/halophenolate ions. Additionally, further halide is released on a time scale of minutes and ho urs. The rates of the conductivity change in the time range from a few ms t o several tens of seconds are proportional to the OH- concentration.