Chemistry of VX 

VX has the chemical name methylphosphonothioic acid, S-[2-[bis(1-methylethyl)amino]ethyl]- O-ethyl ester, and has the molecular formula C 11H 26NO 2PS and formula weight 267.37. Its Chemical Abstracts Service registry number is 50782-69-9.

General Information

The phosphorylthiocholine class of compounds was discovered independently by Ranaji Ghosh of ICI, by Gerhard Schrader of Bayer, and by Lars-Erik Tammelin of the Swedish Institute of Defense Research in 1952-1953. Shortly thereafter, the U.S. Army began a systematic investigation of this class of compounds at Edgewood Arsenal; as a result, VX was developed and stockpiled by the United States. ¹ A short history of the development of nerve gas describes the research and development of the nerve agents. A closely related compound referred to as V-gas was manufactured and stockpiled by the Soviet Union. VX is a colorless and odorless liquid.

Reference

1. Antonov, N., Chemical Weapons at the Turn of the Century, LN 72-96, p. 32.

Noblis recently published a review of melting point, boiling point, vapor pressure as a function of temperature, density as a function of temperature, water solubility, and partitioning data for VX. References to the original data sources are given in the review.

melting point	< -60 deg C
vapor pressure (20 deg C)	0.00015 mm Hg
density (20 deg C)	1.0126 g cm -3
aqueous solubility	miscible below 9.4 deg C
log Kow	0.68
pKa	8.6, 9.12

Additional data on properties, health hazards, and handling is given in the material safety data sheet(MSDS) for VX.

Reference:

1. Bizzigotti, G. O.; Castelly, H.; Hafez, A. M.; Smith, W. H. B.; Whitmire, M. T., Parameters for Evaluation of the Fate, Transport, and Environmental Impacts of Chemical Agents in Marine Environments, Chem. Rev., 2009, in press.

Hydrolysis

The first step in the basic hydrolysis of VX is attack of hydroxide on the phosphorus to form a pentacoordinate phosphorus intermediate:

The pentacoordinate phosphorus intermediate can then decompose in one of two ways. In the first mode, the anion of diisopropylaminoethanethiol is expelled to give the ethyl ester of methylphosphonic acid:

In the second, ethoxide is expelled to give EA2192:

VX also hydrolyzes via displacement of thiophosphonate anion from a carbon atom:

The relative proportions of P-S, P-O, and C-S cleavage are influenced by pH. The reaction described by the k _H2O term predominates at pH < 6.5 and results in P-S cleavage only; the reaction described by the k term predominates in the pH range of 7-10 and results in simultaneous P-S, P-O, and C-S cleavage; and the reaction described by the k _OH predominates at pH > 10 and results in P-S cleavage only. ³ In distilled water, P-S cleavage accounts for 34-37 percent of VX hydrolysis product, P-O cleavage accounts for 42-50 percent of product, and approximately 10 percent of product results from C-S cleavage. ²
Subsequently, both ethyl methylphosphonate undergoes hydrolysis by a similar mechanism (attack of hydroxide on the phosphorus to form a pentacoordinate phosphorus intermediate which collapses expelling a leaving group):

VX Hydrolysis Products
Compound	MW	Solubility, g L-1	Estimated log Kow8
EA2192	239.32	-	1.52
Methylphosphonic acid, ethyl ester	124.08	-	-0.15
Ethanol	46.07	miscible 2	0.28
Diisopropylaminoethanethiol	161.31	-	2.29
Methylphosphonothioic acid, ethyl ester	140.14	-	0.00
Diisopropylaminoethanol	145.25	78.6 3	0.88
Methylphosphonic acid	96.02	"very" 4	-0.70

Noblis recently published a review that compiled hydrolysis rate expressions including activation energies, and an expression for rate constants in sea water for VX. References to the original data sources are given in the review.

 

The hydrolysis chemistry of the Soviet V-gas is expected to be similar. Compounds produced during hydrolysis of the Soviet V-gas are likely to include:

• Methylphosphonothioic acid, S -[2-(diethylamino)ethyl]ester 
• 2-Methyl-1-propanol
• Methylphosphonic acid, 2-methylpropyl ester
• 2-(Diethylamino)ethanethiol
• Methylphosphonic acid
  In addition, VX and V-gas have carbodiimide stabilizers added that produce the following compounds under hydrolytic conditions:
• N,N-Diisopropylurea
• N,N-Dicyclohexylurea

References

1. Estimated using Syracuse Research Corporation, LOGKOW version 1.50; see Meylan, W. M.; Howard, P. H., J. Pharm. Sci. 1995, 84(1): 83-92.
2. The Merck Index, 11 ed., Budavari, S.; O'Niel, M. J.; Smith, A.; Heckelmanm, P. E., Eds., Merck & Co.: Rahway, 1989, p. 594, compound no. 3716.
3. EPA ASTER data base (information available from outreach@du4500.dul.epa.gov).
4. Weast, R. C., Ed., Handbook of Chemistry and Physics, 56th ed., CRC Press, 1976.
5. Bizzigotti, G. O.; Castelly, H.; Hafez, A. M.; Smith, W. H. B.; Whitmire, M. T., Parameters for Evaluation of the Fate, Transport, and Environmental Impacts of Chemical Agents in Marine Environments, Chem. Rev., 2009, in press.

Photolysis

VX and its congeners having the phosphonothiolate structure R-S-P(=O)(R)(OR) are known to undergo reversible photoisomerization to the corresponding phosphonothionates R-O-P(=S)(R)(OR). Other than this, there are no important photochemical processes in sunlight.

Reference:

1. Wyant, R. E.; Slivon, L. E.; Crenshaw, M. D.; Gieseke, J. A., Final Report on Chemical Lists for Analyte Selection, Battelle, 1993, Contract No. 92-H363340-000, Table 1.

Thermolysis

The photoisomerization to the phosphonothionate isomer (above) also can be driven thermally. Other than this, there are no important thermal degradation processes.

Reference:

1. Wyant, R. E.; Slivon, L. E.; Crenshaw, M. D.; Gieseke, J. A., Final Report on Chemical Lists for Analyte Selection, Battelle, 1993, Contract No. 92-H363340-000, Table 1.

Decontamination

VX is hydrolyzed only slowly, and the hydrolysis products include EA2192, which is nearly as toxic as VX itself and longer lived. Thus, hydrolysis-based decontamination schemes are not effective against VX. Oxidation is the method of choice for VX decontamination. ¹ Common bleach (Na ⁺OCl ^-) and superchlorinated bleach (Ca ⁺²(OCl ^-) ₂) react with VX at the N and S atoms: ²

Once the S atom is oxidized, hydrolysis is very rapid: ²

Reactions of VX with peroxide, alkoxide, peroxyacids, and various hydrolysis catalysts were recently reviewed. ³

References

1. Yang, Y.-C.; Baker, J. A.; Ward, J. R., Decontamination of chemical warfare agents, Chem. Rev., 1992, 92, 1729-1743. 2. Yang, Y. C.; Szfraniec, L. L.; Beaudry, W. T.; Rohrbaugh,D. K., Oxidative detoxification of phosphonothiolates, J. Am.Chem. Soc., 1990, 112(18), 6621-6627.
3. Yang, Y. C., Chemical detoxification of nerve agent VX, Acc. Chem. Res., 1999, 32, 109-115.