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 k(S), have been measured at 25 degreesC using competition experiments in the UV/H2O2 process. For a given pair of substrates S-1 and S-2, the relative reactivity beta (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 x 10(9) M-1 s(-1). A quantitative structure-reactivity relationship was found using the Hammett equation. Assuming sigma values to be additive, a value of -0.60 was obtained for the reaction constant rho. This value agrees with the high reactivity and the electrophilic nature of HO. radical. (C) 2003 Elsevier Ltd. All rights reserved.