Chemistry of GA (Tabun) 

Tabun (GA) is a nerve agent. The chemical name of Tabun is dimethylphosphoramidocyanidic acid, ethyl ester; it has the molecular formula C ₅H ₁₁N ₂O ₂P and formula weight 162.13. Its Chemical Abstracts Service registry number is 77-81-6.

General Information

Tabun was originally discovered in 1936 by G. Schrader of I. G. Farben in Germany, ¹ and was first produced industrially in 1942 at Dyhernfurth-am-Oder in Silesia. ²A short history of the development of nerve gas  describes the research and development of the nerve agents in Germany prior to and during World War II. Tabun was used by the Iraqis during the Iran-Iraq war. ³Pure Tabun is a colorless liquid with a fruity odor. ⁴The industrial product has a brownish color and has an odor reminiscent of bitter almonds due to the formation of hydrogen cyanide. Tabun produced by the Germans generally contains 5-20 percent chlorobenzene as solvent and stabilizer. ²

References:
1. The Merck Index, 11 ed., Budavari, S.; O'Niel, M. J.; Smith, A.; Heckelmanm, P. E., Eds., Merck & Co.: Rahway, 1989, p. 1427, compound no. 9001.
2. Antonov, N., Chemical Weapons at the Turn of the Century, report no. LN 72-96, p. 30.
3. Perera, J., New Scientist, 4 April 1985, 107, 8.
4. Franke, S., Manual of Military Chemistry, Volume 1. Chemistry of Chemical Warfare Agents, Deutscher Militîrverlag: Berlin (East), 1967. Translated from German by U.S. Department of Commerce, National Bureau of Standards, Institute for Applied Technology, NTIS no. AD-849 866, pp. 240-241.

Physical Properties of Tabun

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 Tabun. References to the original data sources are given in the review. Additional data on properties, health hazards, and handling is given in the material safety data sheet(MSDS) for tabun.

melting point	-50 deg C
boiling point	246 deg C
vapor pressure (20 deg C)	0.05 mm Hg
density (2 deg C)	1.078 g cm -3
aqueous solubility (0 deg C)	98 g L -1
aqueous solubility (20 deg C) 1	72 g L -1
log K ow	0.38

 

References:
1. Chemical Agent Data Sheets, Vol. I, EO-SR-74001, Edgewood Arsenal, 1974.
2. Estimated using Syracuse Research Corporation, LOGKOW version 1.50; see Meylan, W. M.; Howard, P. H., J. Pharm. Sci.1995, 84(1): 83-92.

Hydrolysis

At pH > 7, Tabun is hydrolyzed by hydroxide anion, producing cyanide anion and monoethyl dimethylphosphoramidate as initial products:

 

At pH < 9 and > 3, Tabun is hydrolyzed by water, producing the same initial products:

 

At pH <5, Tabun is hydrolyzed by acid. The acidic reaction gives protonated dimethylamine and monoethyl phosphorocyanate:

 

The initial hydrolysis products can then be further hydrolyzed to give the following compounds:

 

Tabun Hydrolysis Products
Compound	MW	Solubility, g L-1	Estimated log Kow1
Cyanide (as HCN)	27.03	miscible 2	-0.69
Dimethylphosphoramidic acid, monoethyl ester	153.12	-	-0.04
Dimethylphosphoramidic acid	125.06	-	-0.59
Ethanol	46.07	miscible 3	-0.14
Dimethylamine	45.08	911 4	-0.17
Phosphoric Acid	98.00	5480 4	-

Noblis recently published a review that compiled hydrolysis rate constants, hydrolysis activation energies, and half-lives in sea water for Tabun. References to the original data sources are given in the review.

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. 760, compound no. 4722.
3. Reference 2, p. 594, compound no. 3716.
4. EPA ASTER data base (information available from outreach@du4500.dul.epa.gov).
5. Larsson, L., The hydrolysis of dimethylamido-ethoxy-phosphoryl cyanide (Tabun), Acta Chem. Scand., 1953, 7, 306-314.
6. Langheld, K., Über Ester und Amide der Phosphorsäuren II. Über Versuche zur Darstellung den Lecithenen verwandter Körper., Chem. Ber., 1911, 44, 2076-2087.
7. 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

Tabun and its hydrolysis products exhibit no significant phototransformations in sunlight.

Thermolysis

Tabun and its hydrolysis products are thermally stable at temperatures less than 49 deg C.

Decontamination

Tabun and other G agents are rapidly hydrolyzed in basic solutions, e.g., Na ₂CO ₃, NaOH, or KOH; ¹ GA has a half-life of 1.5 minutes at pH 11 at 25 deg C. ² Several metal ions (Cu ⁺², Pd ⁺², Au ⁺³, Ag ⁺, Ni ⁺², Co ⁺², Zn ⁺²) catalyze the non-enzymatic hydrolysis of Tabun. ³ Little information is available in the literature about decontamination systems effective against Tabun; apparently because it is no longer used by the U.S. or Russia as a major chemical warfare agent. It seems likely that decontamination systems effective with Sarin will also show effectiveness against Tabun.

References:
1. Yang, Y.-C.; Baker, J. A.; Ward, J. R., Decontamination of chemical warfare agents, Chem. Rev., 1992, 92, 1729-1743.
2. Larsson, L., The alkaline hydrolysis of two sarin analogues and of Tabun, Acta Chim. Scand., 1958, 12, 783-785.
3. Augustinsson, K.-B.; Heimburger, G., Enzymatic hydrolysis of organophosphorus compounds. VI. Effect of metal ions on the phosphorylphosphatases of human serum and swine kidney, Acta Chim. Scand., 1955, 9, 383-392.