Mechanisms of abiotic degradation and soil–water interactions of pesticides and other hydrophobic organic compounds. Part 3. Nucleophilic displacement at the phosphorus centre of the pesticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate] by oxygen nucleophiles in aqueous solution: α-effect and mechanism

Abstract

Rate constants for the reaction of the title compound, 1, with a number of oxygen nucleophiles, including structurally related phenoxides and oxygen-based α-nucleophiles, have been measured in aqueous solution at 25 °C. A significant α-effect was observed, confirming participation of the nucleophile in the rate-limiting step of the reaction as well as indicating different transition states (TS) for the reaction of α-nucleophiles compared to normal ones. The Brønsted-type correlation of log k_Nuvs. pK_a of nucleophiles shows a linear plot for the series of structurally related phenoxides in the pK_a range 5.4–10.0, straddling the pK_a of the leaving group (3-methyl-4-nitrophenoxide, pK_a 7.20), but is curved in the highly basic region corresponding to CF₃CH₂O⁻ and HO⁻ as nucleophiles. The slope of the linear portion of the plot (β_Nu) is 0.49 (R² 0.988). The linearity of the plot for the series of structurally related phenoxides is consistent with a concerted mechanism for nucleophilic attack at the P center of the substrate. A value of β_lg −0.39 (R² 0.973) is measured for the reaction of PhO⁻ with substituted phenyl dimethyl phosphorothioate esters. Combining the values of β_Nu and β_lg gives β_eq = 0.88; these parameters when considered together with the effective charge distribution in the TS, demonstrate that the TS for the symmetrical reaction (in which nucleophile = leaving group = 3-methyl-4-nitrophenoxide) has no significant phosphorylium ion character. The Leffler index points to a concerted reaction in which bond formation is slightly ahead of bond rupture in the TS. Data from the present study are compared with literature data for (thio)phosphoryl group transfer. We propose that, unless special structural and/or environmental features prevail, (thio)phosphoryl transfers between phenoxides are more likely to occur via a concerted mechanism. It is shown that the TS for the concerted transfer of (EtO)₂PO between two PhO⁻ moieties shows more pentacoordinate intermediate character than the symmetrical reaction of 1 due to differences in (i) basicity of PhO⁻versus 3-CH₃,4-NO₂PhO⁻, and (ii) abilities of O and S in the PX (X = O, S) moiety to stabilize the incoming negative charge.