Experimental and computational determination of Brønsted coefficients for equilibrium transfer of the O,O‐dimethyl phosphorothioyl group between oxyanion nucleophiles

Abstract

An experimental determination of the β Eq value for equilibrium transfer of the O,O-dimethyl phosphorothioyl group between oxyanion nucleophiles in water and methanol at 25 C is presented. The respective β Eq values in the two solvents are experimentally the same at À1.45AE0.08 and À1.39AE0.12. Based on the observation that the Brønsted correlation for the nucleophilic reaction of phenoxides in water with substrate 1d (dimethyl 4-nitrophenyl phosphorothioate, pK a HOAr of 7.14) is linear over the entire range of phenoxides employed (5.53≤pK a Nu ≤12.38), the reaction for phenoxide nucleophiles displacing phenoxide leaving groups is probably concerted. The obtained data allow one to calculate, for a symmetrical transition state involving 2,4,5-trichlorophenoxide as a nucleophile and leaving group, an approximately 60% P-OAr cleavage and about 40% P-Nuc bond formation. A computational method is presented for the rapid prediction of the β Eq values for such processes in water and methanol, and the results are compared with known values from the literature.

Multiparameter kinetic analysis of alkaline hydrolysis of a series of aryl diphenylphosphinothioates: models for P=S neurotoxins

“…Therefore, the present results are relevant to our understanding of hydrolytic degradation of OP pesticides and to the thio effect that distinguishes between P=O neurotoxins and their P=S analogues. The present findings, combined with previously published results, suggest that other systems, in which the hydrolytic mechanism for OP neurotoxins was presumed to be concerted, likely do proceed via the S N 2(P) route where the leaving groups are good, ie, p K a (LG‐H, the conjugate acid of the leaving group) < 10.…”

“…Through a multiparameter approach to analysis of the kinetics (Hammett, Yukawa‐Tsuno, and Brønsted‐type relationships) and determination of the activation parameters for the alkaline hydrolysis, this study aims to assess the nature and position of the reaction mechanism along the continuum . The relevance of the study to the environmental degradation of P=S OP neurotoxins will also be briefly explored.…”

Linear correlation between effective charge and bond length sensitivity to electronic effects in phosphoryl, sulfonyl, and sulfuryl compounds

“…A β eq value of −0.88 for substituted aryl phosphorothioate triesters was calculated by Buncel et al . This value is substantially lower than the β eq of −1.45 recently obtained for the corresponding dimethylphosphorothioate triesters, calculated from β nuc = 0.57 and β lg = −0.88 . A higher β eq of −1.75 has been obtained theoretically from B3LYP/6‐31++G(d,p) computations using integral equation formalism variant polarizable continuum model (IEFPCM) to model an aqueous environment .…”

“…Here, an effective charge of +0.95 to the bridge oxygen atom of the phosphorothioate triesters is estimated using the slope ( Δr O―X / Δ p K lg ) for the P5 series and Equation . This gives a β eq of −1.95, which is not substantially different from the theoretical β eq value of −1.75 . The predicted effective charge is in agreement with the stronger electronegativity of the P(S)(OR) 2 relative to the P(O)(OR) 2 group, and it is coherent with the concept of additivity.…”

“…This conclusion is not related to reactivity, which was not considered in the linear correlations presented in this paper. However, even though the effective charges differ from the natural charges in that effective charges are hypothetically defined in relation to the standard equilibrium, that is, phenol deprotonation, the atomic charges computed by charges from electrostatic potentials using a grid based method population analysis (CHELPG) have been shown to correlate well with Leffler's α values …”