Hydrolysis of mustard gas

Fig. 2.24. Acceleration of the hydrolysis of mustard gas by neighboring group participation.

El mass spectra of mustard gas (see Table 1) and several derivatives, including its main precursor and hydrolysis product thiodiglycol, have been described (911). The fragmentation of mustard gas... 

This hydrolysis takes place on contact with moisture in the air or with water on the ground and thus mustard gas is slowly destroyed wherever these conditions are encountered the rate of destruction depends upon the amount of water and the temperature. The hydrolysia of mustard gas is also accelerated in the pretfcnce of alkalies, alkaline carbonates, and the solvents of mustard that are miscible with water, such as alcohol. It is rapidly dissolved by the organic solvents, such as ether, chloroform, and acetone, by the light paraffin hydrocarbons, and by all of the organic fats, both animal and vegetable. It is, however, soluble only with difficulty in the mineral oils and in vaseline and paraffin. 

In striking contra.Mt to its marked physical activity in solution and penetration, the chemical activity of mustard gas is rather limited, e.g., its slow hydrolysi.s in contact with water. Its great chemical stability increases the difficulty of decontaminating infected materials with hypochlorites. How ever, mustard reacts violently with the evolution... 

Pham MQ, Harvey SP, Weigand WA et al. (1996). Reactor comparisons for the biodegradation of thiodiglycol, a product of mustard gas hydrolysis. Appl Biochem and Biotechnol 57/58, 779-789. Price CC and von Limbach B (1945). Further data on the toxicity of various CW agents to fish, OSRD No. 5528. Washington DC National Defense Research Committee, Office of Scientific Research and Development. 

The committee expects that combustible gases (carbon monoxide and hydrogen) will be formed in the CST as a result of the reaction between steam and activated carbon. The formation of these gases would reduce the amount of steam available for the hydrolysis of mustard in addition, the gas formed will be highly flammable. This possibility has apparently not been explored to date. 

When mustard gas is kept on the sea-bed, the degree of agitation and current velocity will have an impact on the speed of hydrolysis, too. Mustard gas at the bottom of the sea can survive in oil-like forms. An estimation of the behaviour of sulphur mustard gas indicates a theoretically long, but finite, half-life for the agent in the cold conditions of the ocean deep. Estimations have predicted that a one-tonne block of sulphur mustard gas would take about 5 years to dissolve. Because the dissolution rate is lower than the rate of hydrolysis, it will become the decisive rate. Low temperature and high pressure can provide the proper circumstances for solidification of mustard gas. 

P.D. Bartlett and C.G. Swain, Kinetics of hydrolysis and displacement reactions of (3, fi -dichloro-diethyl sulfide (mustard gas) and of fS-chloro-fi -hydroxydiethyl sulfide (mustard chlorohydrin), J. Am. Chem. Soc., 71, 1406-1415 (1949). 

Despite the ease of hydrolysis, mustard gas may be preserved underground in a solid form for up to ten years. The reason for this is that in an environment where the concentration of water is relatively low, the reaction pathway proceeds to form an intermediate known as thiodiglycol. In a low moisture environment, most of the water available at the solid surface is used in this reaction. Subsequently, another intermediate in the reaction pathway, a sulfonium ion, reacts with the thiodiglycol in the place of water. This reaction then creates stable, non-reactive sulfonium salts, which can act as a protective layer around the bulk of the solid mustard preventing further degradation. 

Although anhydrous mustard is not a substantial corrosion threat to most metals, hydrolysis forms hydrochloric acid and does contribute to mustard s corrosive behavior. An interesting pmr and gas chromatograph-mass spectrometry (GC-MS) study of the hydrolysis of HD has been reported (Logan and Sartori, 2003). In this work, it was shown that hydrolysis (D2O at 22°C) had a half-life of approximately 7 min, but that in the presence of sodium chloride, the half-life increased to approximately 24 min. These results are consistent with those reported by Bartlett and Swain (1949). 

Bartlett, P.D. and Swain, C.G. (1949). Kinetics of the hydrolysis and displacement reactions of p, p -[dichlorodiethyl sulfide (Mustard Gas)] and of P-chloro-p -hyroxidediethyl sulfide (Mustard Chlorohydrin). Journal of the American Chemical Society 71 1406, 1949. 

Ohsawa I, Kanamori-Kataoka M, Tsuge K et al. (2004). Determination of thiodiglycol, a mustard gas hydrolysis product, by gas chromatography-mass spectrometry after tert-butyldimethylsilylation. J Chromatogr B, 1061, 235-241. 

Mustard gas is a liquid which boils at 216°, freezes at 14°, and slowly hydrolyzes when in contact with water. It is the most toxic of the gases used in the war, and in addition is highly vesicant, as it produces painful burns at a high dilution. Its physiological action is thought to be due to the fact that it readily penetrates the cell-walls and then, as the result of hydrolysis, hydrochloric acid is formed. Its action is relatively slow and, as a consequence, the vapor can be breathed for some time without any apparent effect it is for this reason a source of great danger. 

Capacio et al. (2004) have developed a method based on GC/MS to monitor exposure to sulfur mustard relatively at long time frames of 3-6 weeks. Their technique involved the analysis of thiodiglycol formed as a hydrolysis product upon exposure to the mustard gas, HD. Thiodiglycol forms an adduct with blood proteins and is cleaved out from blood proteins upon treatment with NaOH. Thiodiglycol (2, 2 -sulfobisethanol) at trace levels may also be measured by GC coupled to a pulse flame photometric detector (Karvaly et al. 2005). 

Three types of chemical mechanisms have been used for decontamination water/soap wash oxidation and acid/base hydrolysis.9 Mustard (HD) and the persistent nerve agent VX contain sulfur molecules that are readily subject to oxidation reactions. VX and the other nerve agents (tabun [GA], sarin [GB], soman [GD], and GF) contain phosphorus groups that can be hydrolyzed. Therefore, most chemical decontaminants are designed to oxidize mustard and VX and to hydrolyze nerve agents (VX and the G series).1 Water and Water/Soap Wash... 

Mustard gas slowly hydrolyses in water and forms hydrochloric acid and thiodiglycol. Both final products of the hydrolysis are non-toxic. The hydrolysis is dependent on temperature, density, viscosity, pH value and pressure. Because mustard gas is relatively insoluble, the slower dissolving process is the main factor of interest here. There is a huge difference between distilled water and normal sea water. In distilled water the half-life is 8.5 minutes at 25oC, while for salt water at the same temperature the half-life is 60 minutes. For sea water the hydrolysis will be slowed down by a factor of more than 3 times. The pace of hydrolysis of sulphur mustard gas also depends on the content of salt (cations and anions) in an aqueous solution. The reported half-life of sulphur mustard gas in sea-water is 15 minutes at 25oC, 49 minutes at 15oC, and 175 minutes at 5oC. ... 

With respect to the solubility of sulphur mustard gas, the following example seems illustrative. In quiescent water at 18oC, it would take 15 days for half the mass of an agent droplet with an initial diameter of 1 cm to dissolve. This is enough time for other processes to slow or even halt further dissolution and hydrolysis, such as the formation of oligomeric/polymeric layers. 

Any temperature increase will speed up the hydrolysis so for 99 per cent hydrolysis of a saturated mustard gas solution at 20oC, a time of 110 minutes is reported and at 50oC, 4 minutes. Mustard gas is soluble in water with 0.8 g. 1" at 20oC. Once mustard gas is dissolved, it hydrolyses quicker. 

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