Alkaline
Hydrolysis of Organophosphorus Pesticides:
The Dependence of the Reaction Mechanism on the Incoming Group Conformation
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Abstract
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