Peroxyhydrolysis of nerve agent VX and model compounds and related nucleophilic reactions
Abstract
The exceedingly toxic agent VX [O-ethyl
S-(diisopropylaminoethyl) methylphosphonothioate], 1a,
and the midly toxic model compound O,S-diethyl
methylphosphonothioate, 1b, react with HO- to
give parallel P–S and P–O bond cleavages; the
P–O cleavage of VX produces relatively unreactive but very
toxic anionic phosphonothioate. Peroxyhydrolysis of 1a,b with
HO2- involves quantitative P–S
cleavage at rates 30–40 times that with
HO- giving the corresponding phosphonate and
sulfonate ions and disulfide as nontoxic products. In reaction
of 1b with HO2- in
H218O, oxygen in these final products is
not derived from water and HO2-
exclusively displaces the thiolate ion at phosphorus. Reaction
of 1b with HSO5- gives the same
products, but via oxidatively promoted attack of
H218O on phosphorus. Kinetic and isotopic
labelling results on reactions of 1a,b and a range of related
compounds with HO2-,
HO- and RO- and an oximate ion
are interpreted in terms of concerted SN2(P) reactions
rather than stepwise reactions with formation of a trigonal
bipyramidal (TBP) intermediate. Product selectivity depends on
the relative basicities of the anionic nucleophile and
the leaving anions.
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