Comparative reactivity of phosphorotioate insecticides towards N and O nucleophiles.

NATALIA M. ROUGIER; RAQUEL V. VICO; RITA H. DE ROSSI; ELBA I. BUJÁN

Florianópolis

Conferencia; 10th Latin American Conference on Physical Organic Chemistry; 2009

Universidad Federal de Santa Catarina

Fenitrothion (O,O-dimethyl O-(3-methyl-4-nitrophenyl)-phosphorothioate) (1) is a widely used cholinesterase inhibiting insecticide. So, the knowledge of its reactivity towards different nucleophiles is of interest to several areas of environmental chemistry. We have studied the reaction of (1) with HO?, H2O2, NH2OH, N2H4, buthylamine (BuNH2) and piperidine (Pip) in basic media in 2% 1,4-dioxane/H2O at 25ºC and I = 1.00 M (NaCl). Kinetic and spectroscopic evidence have shown that the reaction of HO? and H2O2 proceeds only via the SN2(P) pathway while for NH2O?, NH2OH, N2H4, BuNH2 and Pip competition between SN2(P) and SN2(C) pathways and the hydrolysis reaction was observed (Scheme 1). The percentage of SN2(P) increases with the pH. No evidence for substitution on the aromatic ring was obtained. The UV-visible spectra of the reaction solutions of (1) with N nucleophiles show the formation of demethyl fenitrothion (2) (SN2(C)) and 3-methyl-4-nitrophenol (3) (SN2(P)). The formation of (2) was confirmed by 31P NMR. From kinetic studies it was shown that H2O2, NH2O? present an important alfa effect and that the main reaction pathway for BuNH2 and Pip is the SN2(C).