Competition kinetics using the UV/H2O2 process: a structure reactivity correlation for the rate constants of hydroxyl radicals toward nitroaromatic compounds

The rate constants for hydroxyl radical reaction toward a set of nitroaromatic substrates kS, have been measured at 25 °C using competition experiments in the UV/H2O2 process. For a given pair of substrates S1 and S2, the relative reactivity β (defined as kS1/kS2) was calculated from the slope of the corresponding double logarithmic plot, i.e., of ln[S1] vs. ln[S2]. This method is more accurate and remained linear for larger conversions in comparison with the plots of ln[S1] and ln[S2] against time. The rate constants measured ranged from 0.33 to 8.6 × 109 M−1 s−1. A quantitative structure–reactivity relationship was found using the Hammett equation. Assuming σ values to be additive, a value of −0.60 was obtained for the reaction constant ρ. This value agrees with the high reactivity and the electrophilic nature of HO√ radical.