Chemistry of H (Mustard) 

Mustard "gas", (also known as H, yperite, sulfur mustard, Kampfstoff Lost) is actually a viscous liquid with the chemical name 1,1'Çthiobis[2Çchloroethane], molecular formula C ₄H ₈Cl ₂S, and formula weight 159.08. Its Chemical Abstracts Service registry number is 505-60-2.

General Information

Mustard was first used by the Germans on the night of 12-13 July 1917 near Ypres in Flanders. ¹ The French introduced mustard into their arsenal in June 1918, the British in September 1918. ² In one of the supreme ironies of the history of chemical warfare, the British had tested mustard during the summer of 1916, but the developers had been unable to convince the military of its utility. Meanwhile, the Germans began developing mustard in September 1916, and first filled shells with mustard in the spring of 1917. The Germans waited to introduce mustard to the battlefield until they had accumulated a large supply, knowing that it would be difficult for the Allies to catch up; indeed it took the French 11 months and the British 14 months before they were able to use the agent on the battlefield. ²

Subsequent documented uses of mustard include use in Morocco in 1925, ³ during 1935 in Ethiopia, ³ in China between 1934 and 1944, ³ and in the Iran-Iraq war by both sides. Large quantities were prepared by both the Allies and the Axis during World War II. Although no chemical warfare agents were used in Europe or in the Pacific, ⁴ there was a release of mustard into Bari harbor in Italy in 1943. ³ Mustard was stockpiled by the Soviet Union and the United States through the Cold War.

Mustard was probably first produced by Despretz ⁵ and then later by Riche, ⁶ although neither isolated nor identified the compound. The material was prepared in purer form by Niemann ⁷ and independently by Guthrie, ⁸ both of whom noted the vesicant property of the material. Meyer ⁹ subsequently prepared and established the structure of mustard in 1886. Had some of the earlier workers produced purer material, later commentators believe that they likely would have been severely injured. ¹⁰ Agent-grade material is typically yellow to dark brown; ^4,11 the odor is variously described as "resembling oil of mustard," ⁸ or horseradish, ⁷ or "similar to that of burning garlic," ¹¹ It is a strong vesicant. ⁹

References

1. Paxman, J.; Harris, R., A Higher Form of Killing: The Secret Story of Chemical and Biological Warfare, Hill and Wang: New York, 1982, p. 24.
2. Stockholm International Peace Research Institute, The Problem of Chemical and Biological Warfare. A Study of the Historical Technical, Military, Legal, and Political Aspects of CBW and Possible Disarmament Measures. Vol. 1. The Rise of CB Weapons, Humanities Press: New York, 1971, pp. 47-50.
3. Compton, J. A. F., Military Chemical and Biological Agents: Chemical and Toxicological Properties, Telford Press: Caldwell, NJ, 1988, pp.10-12.
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. 114, 115.
5. Despretz, C. Des Composés triples du chlore, Ann. Chim. Phys., 1822, [1] (21), 437-438.
6. Riche, A. Recherches sur des combinaisons chlorées dérivées des sulfures de méthyle et d’éthyle, Ann. Chim. Phys., 1855, [3] (43), 283-304.
7. Niemann, A. Ueber die Einwirkung des braunen chlorschwefels auf elaylgas, Ann. Chem. Pharm., 1860, 113, 288-292.
8. Guthrie, F., On Some Derivatives from the Olefins, Qly. J. Chem. Soc.,, 1860, 12, 109-126.
9. Meyer, V., Ueber Thiodiglykolverbindungen, Ber., 1886, 19, 3259-3266.
10. Haber, L. F., The Poisonous Cloud. Chemical Warfare in the First World War, Clarendon Press: Oxford, 1986, 342-343.
11. The Merck Index, 11 ed., Budavari, S.; O'Niel, M. J.; Smith, A.; Heckelmanm, P. E., Eds., Merck & Co.: Rahway, 1989, p. 995, compound no. 6225.
12. The Kirk-Othmer Encyclopedia of Science and Technology, 4 ed., Vol. 5, pp. 795-802.

Physical Properties of H.

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, rate of dissolution, and partitioning data for mustard. References to the original data sources are given in the review.

melting point	14.46 deg C
boiling point	216.9 deg C
vapor pressure (20 deg C)	0.08 mm Hg
density (20 deg C)	1.273 g cm -3
aqueous solubility (25 deg C)	0.68 g L -1
aqueous solubility (10 deg C )	0.7 g L -1
estimated log Kow	1.37-2.41

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

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 hydrolysis of H is neighboring group nucleophilic attack of the sulfide S on the ß-carbon to form an intermediate sulfonium ion. ¹

This is considered to be an SN1 reaction with anchimeric ("neighboring group") assistance. The reactant and the ion pair are in equilibrium, so that the observed reaction rate will decrease with increasing chloride concentration. Water attacks the sulfonium ion at one of the ring carbons, opening the ring to give hemimustard and hydrogen chloride:

Hemimustard is also a vesicant. It then reacts in a similar fashion with water to give thiodiglycol and an additional molecule of hydrogen chloride:

The cyclic intermediate formed from hemimustard also reacts via an internal displacement to give 1,4-thioxane and an additional molecule of hydrogen chloride:

The hemimustard hydrolysis reaction gives roughly 4:1 thiodiglycol:thioxane; 1,4Çdithiane and 2-(vinylthio)ethanol are produced from hydrolysis at high concentrations of H (> 2 mole L ^-1) at high temperature (100C) but appear unlikely to form at lower temperatures. ²

H Hydrolysis Products
Compound	FW	Solubility,3g L-1	Estimated log Kow4	product produced from 1 kg H, g
TDG	122.19	6900	-0.62	648
1,4-Thioxane	104.17	286	0.53	120

Noblis recently published a review that compiled hydrolysis rate constants, hydrolysis activation energies, and half-lives in sea water for mustard. References to the original data sources are given in the review. The rates in sea water are considerably slower than the reaction rates in fresh water because aqueous chloride ion affects the equilibrium between mustard and the intermediate sulfonium ion. The effect of chloride is to slow the observed rate of mustard hydrolysis by a factor equal to:

where F varies with the ionic strength according to the Bronsted-Bjerrum rate equation with a limiting value of F⁰_Cl-= 32.2 L mole ^-1 at zero ionic strength. ¹

which uses the constant from the Debye-Hückel equation:

At 25 deg C, the value of the Debye-Hückel A term is calculated as 0.50. The charges ( z _R3S+and z _Cl-) are +1 and -1, and the ionic strength of sea water,  = 0.70, which gives a value of F _Cl- = 4.8 for sea water at 25 deg C. If the hydrolysis rate constant in fresh water is 0.155 min ^-1 at 25 deg C and [Cl ^-] = 0.55 mole L ^-1 this gives a hydrolysis rate constant k ₁= 0.043 min ^-1 in sea water. Although the ionic strength of sea water is above the level at which deviations from the Debye-Hückel law are typically observed, the calculated rate constant corresponds to a half-life of 16.2 min, in close agreement with the measured value from the literature.

References:

1. Bartlett, P. D; Swain, C. G., Kinetics of Hydrolysis and Displacement reactions of b,b'-Dichlorodiethyl Sulfide (Mustard Gas) and of b-Chloro-b-hydroxydiethyl Sulfide (Mustard Chlorohydrin), J. Am. Chem. Soc. 1949, 71, 1406-1415.
2. D'Agostino, P. A.; Provost, L. R., The identification of compounds in mustard hydrolysate (U), DRES Suffield Report 412, Ralston, Alberta, Canada, 1985, available through DTIC AD-A156381, Table III.
3. 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.

Dissolution of Mustard

Despite the relative rapidity of the hydrolysis reaction, H has been found to persist in soil or even under water for periods of decades. In such incidents of long-term persistence, the common thread is the presence of bulk H. While the hydrolysis of dissolved H is relatively fast, the dissolution of H does not occur rapidly. A likely sequence for the fate of bulk H introduced into quiescent water would be the following:

• H that initially dissolves from the droplet is hydrolyzed to TDG.
• At the interface where little water is present, the intermediate sulfonium ion forms and then react with another molecule of H (rather than with water) to form 1,2-bis[(2-chloroethyl)thio]ethane (Q) and 1,2-dichloroethane:

In addition, Q is a byproduct of several methods of mustard agent manufacturing, including the Levinstein process used by the United States. Thus, under long term storage conditions of H, significant additional amounts of Q can accumulate in the container. Note that the presence of a certain level of Q was considered desirable because it is a powerful vesicant in its own right and depresses the freezing point of H. The solubility of Q is 0.3 g L ^Ç1. ¹

• Due to lack of appreciable motion, a concentrated TDG layer builds up at the H droplet-water interface. The TDG at the interface also reacts with the intermediate sulfonium ion to form stable sulfonium salts of the type: ²

• In time, non-reactive sulfonium salts such as these and higher homologues (e.g., from the analogous reactions of Q) build up at the interface between the H droplet and the bulk aqueous phase. The sulfonium salts create a thicker boundary layer.
• Dissolution of H slows, because the driving force for diffusion of H into the bulk aqueous phase decreases. Similarly, diffusion of water into the H droplet slows, which lowers the observed rate of hydrolysis. However, if the water is subject to disturbance, such as a heavy rain, or is rapidly flowing, it is less likely that H droplets would persist for significant periods. References
  1. 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, p. 135.
  2. Yang, Y. C.; Szfraniec, L. L.; Beaudry, W. T.;Ward, R. J., Kinetics and mechanism of the hydrolysis of 2Çchloroethyl sulfides, J. Org. Chem., 1988, 53(14), 3293-3297.
  

Oxidation

H and a number of its hydrolysis products are oxidized to give the sulfoxide and sulfone analogs:

Noteworthy oxidants include oxygen (in air, significant in environmental degradation) and hypochlorite anion (significant in bleach-based decontamination); other oxidants include hydrogen peroxide, nitric acid, potassium permanganate, and chromic acid. The rates of these reactions increase as pH increases. ¹The sulfoxide and sulfone analogs of H are less toxic than the parent compound. H sulfone and H sulfoxide easily eliminates HCl to give divinylsulfone and divinylsulfoxide, respectively. With excess oxidant and heat, the sulfoxide and sulfone are further oxidized to CO ₂, H ₂O, chloride, and SO ₂. Intermediates such as 2-chloroethanesulfonic acid have been isolated, ²but the specific reaction mechanism for further oxidant is not readily apparent.

References

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.
2. 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, p. 127-129.

Photolysis

H and its hydrolysis products exhibit no significant phototransformations in sunlight. The UV spectrum of mustard shows no absorption at wavelengths longer than 300 nm. Irradiation of mustard vapor at 254 nm caused significant degradation, whereas irradiation at 365 nm showed nearly no effect.

Reference:

1. Zuo, G.-M., Z.-X. Cheng, G.-W. Li, L.-Y. Wang, and T. Miao. Photoassisted Reaction of Sulfur Mustard under UV Light Irradiation, Environ. Sci. Technol., 2005, 39(22), 8742-8746.

Thermolysis

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

Decontamination

Dissolved mustard is rapidly hydrolyzed, but dissolution into water is slow. The primary means of detoxification in aqueous solution is via oxidation. Hypochlorite bleaches were the earliest decontaminants used to detoxify mustard. ¹ During World War II both common bleach (NaOCl ^-) and superchlorinated bleaches (Ca(OCl ^-) ₂) were used. More stable N-chloro compounds such as chloramine have been used in more modern personal decontamination systems. In the 1950s a non-aqueous equipment decontamination solution "DS2" (2% NaOH, 70% diethylenetriamine, 28% ethylene glycol monomethyl ether) was developed in which the conjugate base of the glycol ether reacts rapidly with mustard via double elimination. ¹

More recently, decontaminants using Oxone ¹ and various peroxide derivatives ^2,3 have been developed. Oxone and peroxide react with bis(2-chloroethyl) sulfide to give bis(2 chloroethyl) sulfoxide ^1-3. Major products from the decontamination of mustard with bleach include bis(2-chloroethyl) sulfone and (2-chloroethyl)vinyl sulfone ⁴, presumably with the sulfoxide as an intermediate. Minor products include 2-chlorovinyl 2-chloroethyl sulfone, divinyl sulfone, and 2-chlorovinyl vinyl sulfone ⁴.

References:

1. Yang, Y.-C.; Baker, J. A.; Ward, J. R., Decontamination of chemical warfare agents, Chem. Rev., 1992, 92, 1729-1743.
2. Wagner, G. W., Sorrick, D. C.; Procell, L. R.; Brickhouse, M. D.; McVey, I. F.; Schwartz, L. I., Decontamination of VX, GD, and HD on a Surface Using Modified Vaporized Hydrogen Peroxide, Langmuir, 2007, 23(3), 1178-1186.
3. Wagner, G. W.; Yang, Y.-C., Rapid Nucleophilic/Oxidative Decontamination of Chemical Warfare Agents, Ind. Eng. Chem. Res.., 2002, 41(8), 1925-1928.
4. Samuel, J. B.; Beaudry, W. T.; Rohrbaugh, D. K.; Szafraniec, L. L.; Butrow, A. B.; Procell, L. R.; Sorrick, D. C.; Yang, Y.-C., Agent Neutralization Study II: Detoxification of HD with Aqueous Bleach, ERDEC-TR-458, Edgewood Research Development and Engineering Center, 1998, DTIC accession no. AD-A339448.