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: alpha-effect and mechanism

Rate constants for the reaction of the title compound, 1, with a number of oxygen nucleophiles, including structurally related phenoxides and oxygen-b ased alpha -nucleophiles, have been measured in aqueous solution at 25 degr eesC. A significant alpha -effect was observed, confirming participation of the nucleophile in the rate-limiting step of the reaction as well as indic ating different transition states (TS) for the reaction of alpha -nucleophi les compared to normal ones. The Bronsted-type correlation of log k(Nu) vs. pK(a) of nucleophiles shows a linear plot for the series of structurally r elated 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 (beta (Nu)) is 0.49 (R-2 0.988). T he linearity of the plot for the series of structurally related phenoxides is consistent with a concerted mechanism for nucleophilic attack at the P c enter of the substrate. A value of beta (lg) -0.39 (R-2 0.973) is measured for the reaction of PhO- with substituted phenyl dimethyl phosphorothioate esters. Combining the values of beta (Nu) and beta (lg) gives beta (eq) = 0 .88; these parameters when considered together with the effective charge di stribution in the TS, demonstrate that the TS for the symmetrical reaction (in which nucleophile = leaving group = 3-methyl-4-nitrophenoxide) has no s ignificant phosphorylium ion character. The Leffler index points to a conce rted 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 p henoxides are more likely to occur via a concerted mechanism. It is shown t hat the TS for the concerted transfer of (EtO)(2)P=O between two PhO- moiet ies shows more pentacoordinate intermediate character than the symmetrical reaction of 1 due to differences in (i) basicity of PhO- versus 3-CH3,4-NO2 PhO-, and (ii) abilities of O and S in the P=X (X = O, S) moiety to stabili ze the incoming negative charge.