Autocatalytic Hydrolysis of V-Type Nerve Agents

Abstract

Both V-type nerve agents MeP(O)(OR)(SCH2CH2NR‘2), VX (R = C2H5; R‘ = i-C3H7) and its isomeric analog RVX (the “Russian V-agent”, R = i-C4H9; R‘ = C2H5), react slowly but completely with an equimolar amount of H2O via exclusive P−S cleavage to produce the corresponding phosphonic acid (MeP(O)(OR)OH) and 2-aminoethanethiol (HSCH2CH2NR‘2). The reaction is believed to be initiated by nucleophilic attack of the deprotonated phosphonic acid on the protonated V-agent to produce a diphosphonate intermediate ((MeP(O)(OR))2O) that rapidly hydrolyzes to regenerate the phosphonic acid. The autocatalytic ionic chain reaction is thus continued in the nearly nonaqueous reaction medium. The viscous final product mixture remains reactive toward freshly added trace amounts of the V-agent, giving the same final reaction half-life of 13−15 h at 23 °C. When water is insufficient and depleted, the diphosphonate intermediate accumulates and reacts with the aminoethanethiol to regenerate the V-agent. This autocatalytic hydrolysis process is not observed with a simpler phosphonothioate analog (MeP(O)(OC2H5)(SC2H5)), which suggests that the attack of the phosphonic acid on the V-agent is intramolecularly assisted by the protonated amino group.