Mechanism of Hydroxyl Radical‐Induced Breakdown of the Herbicide 2,4‐Dichlorophenoxyacetic Acid (2,4‐D)

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Abstract
Oxidative transformations by the hydroxyl radical are significant in advanced oxidation processes for the breakdown of organic pollutants, yet mechanistic details of the reactions are lacking. A combination of experimental and computational methods has been employed in this study to elucidate the reactivity of the hydroxyl radical with the widely used herbicide 2,4‐D (2,4‐dichlorophenoxyacetic acid). The experimental data on the reactivity of the hydroxyl radical in the degradation of the herbicide 2,4‐D were obtained from γ‐radiolysis experiments with both 18O‐labeled and unlabeled water. These were complemented by computational studies of the .OH attack on 2,4‐D and 2,4‐DCP (2,4‐dichlorophenol) in the gas phase and in solution. These studies firmly established the kinetically controlled attack ipso to the ether functionality as the main reaction pathway of .OH and 2,4‐D, followed by homolytic elimination of the ether side chain. In addition, the majority of the early intermediates in the reaction between the hydroxyl radical and 2,4‐DCP, the major intermediate, were identified experimentally. While the hydroxyl radical attacks 2,4‐D by .OH‐addition/elimination on the aromatic ring, the oxidative breakdown of 2,4‐DCP occurs through .OH addition followed by either elimination of chlorine or formation of the ensuing dichlorophenoxyl radical.

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