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

摘要

We report herein the first kinetic study of a P=S containing organophosphorous 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 (k_(obs) 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-S_E2 process.A putative A-2 mechanism in this region is supplanted by the proposed A-S_E2 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).