Nucleophilic Aliphatic Substitution Reactions of Propachlor, Alachlor, and Metolachlor with Bisulfide (HS^-) and Polysulfides (S_n^2-)

Abstract

Reactions of bisulfide and polysulfides with alachlor, propachlor, and metolachlor were examined in aqueous solution to investigate the role reduced sulfur species could play in effecting abiotic transformations of chloroacetanilide herbicides. Experiments at 25 degrees C demonstrated that reactions were approximately first-order in HS- concentration and revealed that polysulfides are considerably more reactive than HS-. delta H not equal to values for reactions of the three chloroacetanilides with HS- are statistically indistinguishable at the 95% confidence level, as are delta S not equal to values, despite significant differences in second-order rate constants (kHS-). Transformation products were characterized by GC/MS (in some cases following methylation) and were found to be consistent with substitution of chlorine by the sulfur nucleophile. Products containing multiple sulfur atoms were observed for the reactions of chloroacetanilides with polysulfides, while products resulting from reaction with HS- only possessed a single sulfur atom. When second-order rate constants at 25 degrees C are multiplied by HS- and polysulfide concentrations reported in salt marsh porewaters, predicted half-lives range from minutes to hours. HS- and especially polysulfides could thus exert a substantial influence on the fate of chloroacetanilide herbicides in aquatic environments. Oxidation of the resulting sulfur-substituted products could generate ethane sulfonic acid derivatives, previously reported as prevalent chloroacetanilide degradates.

Heterogeneous Nucleophilic Transformation of Metolachlor by Bisulfide on Alumina Surface

“…The bisulfide is a strong nucleophile and can result metolachlor an efficient transformation rate, in which the complete transformation was achieved at the reaction time of 28 days in the presence of 3 mM bisulfide, with the pseudo-first-order kinetic constant value k of 0.112 AE 0.0082 days À1 (R 2 ¼ 0.970). The efficient transformations of chloroacetanilide herbicides, including metolachlor, by bisulfide were also reported in the previous report [13]. However, the transformation rate of metolachlor in our study was significantly higher than that reported by Loch et al [13], with the calculated second order rate constant of 1.0 Â 10 À2 M À1 s À1 compared with 0.25 Â 10 À2 M À1 s À1 , which may resulted from the reaction systems with different buffer solution for formation rate of bisulfide from Na 2 S.…”

“…The efficient transformations of chloroacetanilide herbicides, including metolachlor, by bisulfide were also reported in the previous report [13]. However, the transformation rate of metolachlor in our study was significantly higher than that reported by Loch et al [13], with the calculated second order rate constant of 1.0 Â 10 À2 M À1 s À1 compared with 0.25 Â 10 À2 M À1 s À1 , which may resulted from the reaction systems with different buffer solution for formation rate of bisulfide from Na 2 S.…”

“…Bisulfide is one type of active reduced sulfur species and can act as a strong nucleophilic reagent for accelerating herbicide transformation [13]. This compound also plays an important role in the transformation of chloracetanilide herbicides, especially in the heterogeneous reaction systems with the presences of aluminas.…”

“…The natural sulfur species are important nucleophiles and have received increasing attention in recent years for their roles in accelerating herbicide transformation [11,12]. The reduced sulfur species, e.g., HS À , S n , are reportedly active environmental ''reagents'' that can react with a wide array of organic pollutants, including chloroacetanilide herbicides, which undergo nucleophilic substitution and dehalogenation processes [12][13][14][15][16]. However, all these previous studies were conducted in homogeneous conditions.…”

Dechlorinating transformation of propachlor through nucleophilic substitution by dithionite on the surface of alumina

“…Because of this resistance to degradation, exploiting the reductive transformation of chloroacetanilides through nucleophilic substitution in soil has recently received considerable attention (Lippa et al 2004). Because of the high reducing potential and strong nucleophilic abilities, the sulfur compounds, such as biosulfate (Bian et al 2009), thiosulfate (Gan et al 2002;Cai et al 2007), and polysulfide (Loch et al 2002), have been reported to readily dehalogenate organic halogenated pollutants efficiently. Dithionite is a relatively inexpensive sulfur compound which has long been used extensively in industrial applications (de Carvalho and Schwedt 2002).…”