On the reaction of 2,4,5-trichlorophenol with hydroxyl radicals: New information on transients and their properties

A reinvestigation of the ^̇ OH radical reaction with 2,4,5-trichlorophenol (TCP) provided unambiguous direct evidence, in contrast to an earlier study, for the formation of two different ^̇ OH-adducts, namely, at the C-6 and C-3 positions. They appear to be formed at about equal yield and exhibit different optical absorption spectra with maxima at 320 and 350 nm for the C-6 adduct and 320 nm for the C-3 adduct. Both are mild reductants as can be deduced from their reaction with Fe(CN) ₆ ^3- . Owing to its relatively low pK _a (4.8 ± 1), the C-6-adduct reacts in neutral to slightly acid solutions preferentially through its deprotonated form. Absolute rate constants have been measured for the reduction of Fe(CN) ₆ ^3- by the anion of the 6-hydroxy adduct radical (1.4 ± 0.3) × 10 ⁸ M ^-1 s ^-1 and by the neutral form of the 3-hydroxy adduct radical (2.7 ± 0.6) × 10 ⁶ M ^-1 s ^-1 . The latter assignment corrects the previous conclusion that attributed the low 10 ⁶ M ^-1 s ^-1 order of magnitude rate constant to the reaction of the C-6 adduct radical. Based on the reduction kinetics measurements, the rate constant for the C-6 adduct radical deprotonation process has been estimated to be about 3 × 10 ⁴ s ^-1 . The C-6-adduct, both in its neutral as well as in its anionic form, gains particular stability through hydrogen bond bridging between the two hydroxyl groups positioned at C-6 and C-1, and this accounts for the elevated rates that we report.