Concerted rate-limiting proton transfer to sulfur with nucleophilic attack at phosphorus — A new proposed mechanism for hydrolytic decomposition of the P=S pesticide, Diazinon, in moderately acidic sulfuric acid media

Abstract

We report herein the first kinetic study of a P=S containing organophosphorus pesticide, Diazinon (1), in the moderately concentrated acid region. Product analyses (31P NMR) show that reaction occurs only at the P centre. The rate-acidity profile (kobs vs. molarity of H2SO4) appears as a curve in which the initial slight downward trace (molarity = 1 to ca. 5) is followed by sharper upward curve (molarity ca. 5 to 14). Using treatments involving the excess acidity (X) method, the A-1 and A-2 mechanistic possibilities were found to be inoperative over the full acidity range. A novel mechanism is proposed for the higher acidity (X ca. 2–6) region. This mechanism involves proton transfer to P=S from hydronium ion with concomitant proton transfer from water, which effectively delivers hydroxide to the P centre in a variant of the A-SE2 process. A putative A-2 mechanism in this region is supplanted by the proposed A-SE2 variant where the cyclic array results in proton transfer being efficiently coupled with nucleophilic attack involving water. This constitutes the first report of rate-limiting proton transfer at the P=S functionality in acid hydrolysis of this class of organophosphorus neutroxins. A 600 000-fold acceleration in the decomposition of Diazinon is associated with the change of medium from neutral aqueous solution to the most acidic medium studied (X ca. 6). Key words: phosphorothioate ester hydrolysis, acid catalysis, rate-limiting proton transfer at P=S, excess acidity analysis, new A-SE2 variant mechanism.

Transition from concerted to stepwise processes as a function of leaving group ability: density functional theory and experimental study of lyoxide‐promoted cleavages of phosphorothioate and phosphate triesters in water and methanol

“…31 P NMR spectra of the starting triesters as well as that of the authentic trimethyl phosphate and trimethyl phosphorothioate in methanol were used as references (the chemical shifts of the known compounds in CD 3 OD were referenced using an inserted sealed glass tube containing ~0.1 M phosphoric acid in CD 3 OD). A competitive demethylation process, resulting from a nucleophilic attack on one of the CH 3 groups to form X═P(OR)(OCH 3 )(O − ), was observed for substrates 1n , o and 2m , n , p under basic conditions in methanol. The reaction progress was monitored by determining both the % conversion of the starting triester and the ratios of the P–OLg cleavage product (trimethyl phosphate/phosphorothioate triester) to the demethylation product (a diester).…”

Hydrolysis Mechanisms for the Organopalladium Complex [Pd(CNN)P(OMe)3]BF4 in Sulfuric Acid

“…The k 0,ASE2 rate constants deduced for the rate-determining step at several temperatures are given in Table . To our knowledge, no upper limit for the k 0,ASE2 value has in principle been reported in literature for the wide variety of reactions that follow the A-S E 2 mechanism. ,− …”

“…To our knowledge, no upper limit for the k 0,ASE2 value has in principle been reported in literature for the wide variety of reactions that follow the A-S E 2 mechanism. 22,[38][39][40] The mechanism put forward is outlined in Scheme 1. In a previous work, we calculated the dissociation constant pK SH 2+…”

Multiparameter kinetic analysis of alkaline hydrolysis of a series of aryl diphenylphosphinothioates: models for P=S neurotoxins

“…In this study we have found that the OP simulants, aryl diphenylphosphinothioates, reacted with hydroxide, as nucleophile, via an S N 2(P) mechanism. Of the environmental pathways for degradation of OP neurotoxins, including photochemical and microbial routes, hydrolysis remains a major abiotic contributor to OP decomposition. Water is, of course, the ubiquitous solvent and nucleophile, and under alkaline conditions (eg, calcareous fresh waters) hydroxide is the major environmental nucleophile.…”

“…Under such optimal conditions impressive catalytic enhancements for methanolysis of OP pesticides have been reported, ranging from 20,000‐fold to millions‐fold, and to billion‐fold with La 3 + (bridged methoxide lanthanum dimers) and from million‐fold to 100 billion‐fold with Cd 2 + , Mn 2 + , and a Pd 2 + complex . The effect of sulfuric acid media for decomposition of the organophosphorothioate insecticide, diazinon, gives significant acceleration of decomposition relative to pure water …”