Role of the P?F Bond in Fluoride-Promoted Aqueous VX Hydrolysis: An Experimental and Theoretical Study

摘要

Following our ongoing studies on the reactivity of thenfluoride ion toward organophosphorus compounds, we established that thenextremely toxic and environmentally persistent chemical warfare agentnVX (O-ethyl S-2-(diisopropylamino)ethyl methylphosphonothioate) isnexclusively and rapidly degraded to the nontoxic product EMPA (ethylnmethylphosphonic acid) even in dilute aqueous solutions of fluoride. Thenunique role of the P−F bond formation in the reaction mechanism wasnexplored using both experimental and computational mechanistic studies. Innmost cases, the “G-analogue” (O-ethyl methylphosphonofluoridate, Et-G)nwas observed as an intermediate. Noteworthy and of practical importance isnthe fact that the toxic side product desethyl-VX, which is formed innsubstantial quantities during the slow degradation of VX in unbuffered water,nis completely avoided in the presence of fluoride. A computational study onna VX-model, O,S-diethyl methylphosphonothioate (1), clarifies the distinctive tendency of aqueous fluoride ions to react with suchnorganophosphorus compounds. The facility of the degradation process even in dilute fluoride solutions is due to the increasednreactivity of fluoride, which is caused by the significant low activation barrier for the P−F bond formation. In addition, the uniquennucleophilicity of fluoride versus hydroxide toward VX, in contrast to their relative basicity, is discussed. Although the reactionnoutcomes were similar, much slower reaction rates were observed experimentally for the VX-model (1) in comparison to VX.