OH-Radical Induced Oxidation of Phenoxyacetic Acid and 2,4-Dichlorophenoxyacetic Acid. Primary Radical Steps and Products

Abstract

The reactions of •OH radicals with 2,4-dichlorophenoxyacetic acid (2,4-D), k = (6.6 ± 0.5) × 109 M-1 s-1, phenoxyacetic acid (PAA), k = (10 ± 1) × 109 M-1 s-1, and 2,4-dichlorophenol (2,4-DCP), k = (7.1 ± 0.5) × 109 M-1 s-1, have been studied using pulse and gamma radiolysis. The •OH adducts produced by addition to the ring at positions not occupied by substituted groups were oxidized by K3[Fe(CN)6], with the corresponding hydroxylated species analyzed by HPLC. The distribution of •OH radical addition to the aromatic ring of 2,4-D is C1, 17%; C2/C4, 20%; C3/C5/C6, 47%; and for •OH reaction with PAA, it is C1, 5.5%; C2/C6, 45%; C3/C5, 11%; C4, 36%. Whereas the hydroxycyclohexadienyl radicals of 2,4-D have no observable reactivity toward oxygen, those of PAA and 2,4-DCP do react, with k(•OH adduct−PAA) + O2 = (5.1 ± 0.2) × 108 M-1 s-1 and k(•OH adduct−2,4-DCP) + O2 = (1.1 ± 0.1) × 108 M-1 s-1. The phenoxyl radicals formed by chloride elimination from the ipso-chloro-•OH adducts of 2,4-D rapidly oxidize TMPD to TMPD•+, k = (3.6 ± 0.5) × 109 M-1 s-1.