engleski

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

A re-investigation of the &middot ; 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 &middot ; 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)63-. Owing to its relatively low pKa (4.8 &plusmn ; 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)63- by the anion of the 6-hydroxy adduct radical (1.4 &plusmn ; 0.3) × 108 M-1 s-1, and by the neutral form of the 3-hydroxy adduct radical (2.7 &plusmn ; 0.6) × 106 M-1 s-1. The latter assignment corrects the previous conclusion that attributed the low 106 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 × 104 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.

free radicals; rate constants

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