Alkali-Metal Ion Catalysis and Inhibition in Nucleophilic Displacement Reaction of O-Ethyl O-4-Nitrophenyl Phenylphosphonothioate with Alkali-Metal Ethoxides

Abstract

A kinetic study on nucleophilic displacement reactions of O-ethyl O-4-nitrophenyl phenylphosphonothioate (an insecticide called EPN) with alkali-metal ethoxides (EtOM; M = Li, Na, K, and K/18-crown-6-ether) is reported. Dissection of pseudo-first-order rate constant (kobsd) into the second-order rate constants for the reaction of EPN with the dissociated EtO− (kEtO-) and ion-paired EtOM (kEtOM) has revealed that the reactivity increases in the order kEtOLi < kEtO- < kEtONa < kEtOK < kEtOK/18-crown-6-ether, indicating that Li+ inhibits the reaction while the other M+ ions behave as a Lewis acid catalyst in the order Na+ < K+ < 18C6-complexed K+. The contrasting M+ ion effects have been explained in terms of the hard and soft acids and bases (HSAB) principle, since EPN, as a polarizable P=S centered electrophile, would exert a weak interaction with Li+ (a hard acid) but a strong interaction with the 18C6-complexed K+ ion (a soft acid). M+ ions catalyze the current reaction by increasing the electrophilicity of the reaction center. EPN is less reactive than O-4-nitrophenyl diphenylphosphinothioate but is more reactive than O-4-nitrophenyl O,O-diethyl thiophosphate. Factors governing the reactivity of these P=S centered electrophiles are discussed.