Abiotic Reduction Reactions of Dichloroacetamide Safeners: Transformations of “Inert” Agrochemical Constituents

• Published in: Environmental science & technology 2012-02, Vol.46 (4), p.2187-2195
• Main Authors: Sivey, John D, Roberts, A. Lynn
• Format: Article
• Language: English
• Subjects: Acetamides - chemistry; Agronomy. Soil science and plant productions; Applied sciences; Biological and medical sciences; Biological and physicochemical phenomena; Biological and physicochemical properties of pollutants. Interaction in the soil; Chemical reactions; Classification; Crops; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Ferric Compounds - chemistry; Fundamental and applied biological sciences. Psychology; Herbicides; Herbicides - chemistry; Hydrogen; Iron - chemistry; Iron Compounds - chemistry; Kinetics; Minerals - chemistry; Natural water pollution; Oxidation-Reduction; Pollution; Pollution, environment geology; Soil and sediments pollution; Soil and water pollution; Soil science; Water treatment and pollution
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es203755h

Safeners are so-called “inert” constituents of herbicide formulations added to protect crops from the toxic effects of herbicides. We examined the reactivity of three dichloroacetamide safeners and 12 structural analogues [all neutral compounds of the form Cl2CXC(O)NRR′; X = H, Cl; R-groups include alkyl, branched alkyl, n-allyl, and cyclic moieties] in one homogeneous and two heterogeneous reductant systems: solutions of Cr(H2O)6 2+, suspensions of FeII-amended goethite, and suspensions of FeII-amended hematite. Analyses of reaction products indicate each safener can undergo stepwise hydrogenolysis (replacement of chlorine by hydrogen) in each system at near-neutral pH. The first hydrogenolysis step generates compounds similar (in one case, identical) to herbicide active ingredients. Rates of product formation and (when reactions were sufficiently fast) parent loss were quantified; reaction rates in heterogeneous systems spanned 2 orders of magnitude and were strongly influenced by R-group structure. The length of n-alkyl R-groups exerted opposite effects on hydrogenolysis rates in homogeneous versus heterogeneous systems: as R-group size increased, reduction rates in heterogeneous systems increased, whereas reduction rates in the homogeneous system decreased. Branched alkyl R-groups decreased hydrogenolysis rates relative to their straight-chain homologues in both homogeneous and heterogeneous systems. Reaction rates in heterogeneous systems can be described via polyparameter linear free energy relationships employing molecular parameters likely to influence dichloroacetamide adsorption. The propensity of dichloroacetamide safeners to undergo reductive transformations into herbicide-like products challenges their classification as “inert” agrochemical ingredients.