Competition kinetics using the UV/H 2O 2 process: a structure reactivity correlation for the rate constants of hydroxyl radicals toward nitroaromatic compounds

Abstract

The rate constants for hydroxyl radical reaction toward a set of nitroaromatic substrates k S, have been measured at 25 °C using competition experiments in the UV/H 2O 2 process. For a given pair of substrates S 1 and S 2, the relative reactivity β (defined as k S1/ k S2) was calculated from the slope of the corresponding double logarithmic plot, i.e., of ln[S 1] vs. ln[S 2]. This method is more accurate and remained linear for larger conversions in comparison with the plots of ln[S 1] and ln[S 2] against time. The rate constants measured ranged from 0.33 to 8.6 × 10 9 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.