Alkaline Hydrolysis of Organophosphorus Pesticides: The Dependence of the Reaction Mechanism on the Incoming Group Conformation
journal contributionposted on 03.07.2014, 00:00 by Edyta Dyguda-Kazimierowicz, Szczepan Roszak, W. Andrzej Sokalski
The fundamental mechanism of organophosphate hydrolysis is the subject of a growing interest resulting from the need for safe disposal of phosphoroorganic pesticides. Herein, we present a detailed ab initio study of the gas-phase mechanisms of alkaline hydrolysis of P–O and P–S bonds in a number of organophosphorus pesticides, including paraoxon, methyl parathion, fenitrothion, demeton-S, acephate, phosalone, azinophos-ethyl, and malathion. Our main finding is that the incoming group conformation influences the mechanism of decomposition of organophosphate and organothiophosphate compounds. Depending on the orientation of the attacking nucleophile, hydrolysis reaction might follow either a multistep pathway characterized by the presence of a pentavalent intermediate or a one-step mechanism proceeding through a single transition state. Despite a widely accepted view of the phosphotriester P–O bonds being decomposed exclusively via a direct-displacement mechanism, the occurrence of alternative, qualitatively distinct reaction pathways was confirmed for alkaline hydrolysis of both P–O and P–S bonds. As the pesticides included in our quantum chemical analysis involve organophosphate, phosphorothioate, and phosphorodithioate compounds, the influence of oxygen to sulfur substitution on the structural and energetic characteristics of the hydrolysis pathway is also discussed.
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Organophosphorus Pesticidesgroup conformation influenceshydrolysis reactionphosphorodithioate compoundsdirect-displacement mechanismmechanism proceedinggas-phase mechanismsmethyl parathiontransition statehydrolysis pathwayreaction pathwaysmultistep pathwaybondquantum chemical analysisorganophosphorus pesticidesIncoming Group Conformationorganophosphate hydrolysisphosphoroorganic pesticidesorganothiophosphate compoundssulfur substitutionAlkaline HydrolysisReaction Mechanismab initio study