Acceleration of Nucleophilic Attack on an Organophosphorothioate Neurotoxin, Fenitrothion, by Reactive Counterion Cationic Micelles. Regioselectivity as a Probe of Substrate Orientation within the Micelle

Abstract

31P NMR and UV-vis spectrometric evidence has revealed an unexpected regioselectivity in the reaction of fenitrothion, 1, an organophosphorus pesticide, with the cetyltrimethylammonium (CTA) surfactants CTAOH and CTAMINA, that incorporate the reactive counterions OH- and MINA- (the anti-pyruvaldehyde 1-oximate anion). While both micellar solutions accelerate decomposition of 1 compared to aqueous OH- alone, CTAMINA produced the largest rate enhancement (ca. 10(5)) at a pH (8.39) appropriate for environmental applications. In the absence of surfactant, reaction proceeds solely via the SN2(P) pathway. In the presence of surfactant but below the critical micelle concentration (cmc), a competitive SN2(C) pathway was observed in addition to SN2(P). Above the cmc, however, the CTAOH reaction again proceeded solely via the SN2(P) pathway while both pathways were operative with CTAMINA. The changes in reactivity and mechanistic pathway are discussed in terms of premicellar and micellar influences on rates and regioselectivity. A proposal that would account for the observed regioselectivity in the micellar system is that the aromatic ring and aliphatic side-chains of 1 are oriented toward the micellar interior, while the P=S moiety faces the aqueous pseudophase.

Comparative study of nucleophilic efficacy of pralidoxime towards phosphorus, sulfur and thiophosphorus based esters

“…It has been already proved that cationic surfactants accelerate the reaction. According to Buncel et al [26], the electrostatic attraction of the cationic head groups of the surfactants at the micelle surface to the nucleophilic anion counterions leads to augmentation of the local concentration of the nucleophile, whereas incorporation of the substrate in the micelle leads to a higher local concentration of There is little information about the effect of the variation of head groups on the micellar structure [27][28][29].…”

Nucleophilic debenzoylation of p‐nitrophenyl benzoate in cationic micellar media

“…The hydroxamate ion concentration in the vicinity of the micellar surface is expected to increase with an increase in the aggregation number. According to Buncel et al [19], the electrostatic attraction of the cationic head groups of the surfactants at the micelle surface to the nucleophile anion counterion leads to augmentation of the local concentration of the nucleophile, whereas incorporation of the substrate in micelle leads to a higher local concentration of the substrate. This enhanced concentration of the reactants accounts for the higher rate of reaction.…”

Antibacterial activity of di-2-ethylaniline phosphate screened by paper disc diffusion method

“…The discovery of the acute toxicity of various pentavalent organophosphorus compounds toward living species led to the development, industrial production and widespread use of phosphoric, thiophosphoric and phosphonothioic acid derivatives as biocides for animal and crop protection (Morales et al, 2002;Toy et al, 1987). Due to the wide importance of these esters their biological significance has been extensively investigated over the past decades (Balakrishnan et al, 2004;Cox et al, 1964).…”