title: Nucleophilic substitution reactions of chlorpyrifos-methyl with sulfur species.
abstract: Chlorpyrifos-methyl is widely used in the control of insects on certain stored grain, including wheat, barley, oats, rice, and sorghum. The reactions of chlorpyrifos-methyl with hydrogensulfide/bisulfide (H2S/HS-), polysulfides (Sn(2-)), thiophenolate (PhS-), and thiosulfate (S2O3(2-)) were examined in well-defined aqueous solutions over a pH range from 5 to 9. The rates are first-order in the concentration of the different reduced sulfur species. The resulting data indicate that chlorpyrifos-methyl undergoes a S(N)2 reaction with the reduced sulfur species. The transformation products indicate that the nucleophilic substitution of reduced sulfur species occurs at the carbon atom of a methoxy group to form the desmethyl chlorpyrifos-methyl. The formation of trichloropyridinol, a minor degradation product, could be attributed entirelyto hydrolysis. The reaction of chlorpyrifos-methyl with thiophenolate leads to the formation of the corresponding methylated sulfur compound. The resulting pseudo-first-order rate constant for chlorpyrifos-methyl with bisulfide yielded a second-order rate constant of 2.2 (+/- 0.1) x 10(-3) M(-1) s(-1). The determined second-order rate constants show that the reaction of chlorpyrifos-methyl with HS- is of the same order of magnitude as the reaction of chlorpyrifos-methyl with S2O3(2-) with a second-order rate constant of 1.0 (+/- 0.1) x 10(-3) M(-1) s(-1). The second-order rate constant for chlorpyrifos-methyl with polysulfides (3.1 (+/- 0.3) x 10(-2) M(-1) s(-1)) is of the same order of magnitude as the one with thiophenolate (2.1 (+/- 0.2) x 10(-2) M(-1) s(-1)). The second-order rate constant for the reaction of polysulfides is approximately 1 order of magnitude greater than that for the reaction with HS-. When the determined second-order rate constants are multiplied by the concentration of HS-, polysulfides and thiosulfate reported in salt marshes and porewaters, predicted half-lives show that the inorganic reduced sulfur species present at environmentally relevant concentrations may represent an important sink for phosphorothionate triesters in coastal marine environments.
authors.affiliation: Chemistry Department, The City College of The City University of New York, New York, New York 10031, USA.
authors.forename: Tong
authors.initials: T
authors.lastname: Wu
authors.forename: Urs
authors.initials: U
authors.lastname: Jans
mesh_terms: D004390:Chlorpyrifos / Q000031:analogs & derivatives* / Q000737:chemistry; D019015:Geologic Sediments / Q000737:chemistry; D006207:Half-Life; D006863:Hydrogen-Ion Concentration; D007306:Insecticides / Q000737:chemistry*; D007700:Kinetics; D013455:Sulfur / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis
chemical_list: D007306:Insecticides; D014874:Water Pollutants, Chemical; D013455:Sulfur; D004390:Chlorpyrifos; C007031:chlorpyrifos-methyl