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Sulfur mustard

The sulfur mustards, of which mustard gas (bis(2-chloroethyl) sulfide) is a member, are a class of related cytotoxic, vesicant chemical warfare agents with the ability to form large blisters on exposed skin. In their pure form most sulfur mustards are colorless, odorless, viscous liquids at room temperature. When used in impure form as warfare agents they are usually yellow-brown in color and have an odor resembling mustard plants, garlic or horseradish, hence the innocuous name. The name originally assigned by the German Empire was "Lost", after the individuals Lommel and Steinkopf who first proposed the military use of Sulfur Mustard to the German Imperial General Staff.

Mustard agents are regulated under the 1993 Chemical Weapons Convention (CWC). Three classes of chemicals are monitored under this Convention, with sulfur and nitrogen mustard grouped in Schedule 1, as substances with no use other than chemical warfare.

Formulations

In its history, various types and and mixtures of sulfur mustard have been employed. These include:

H – Also known as HS ("Hun Stuff") or Levinstein mustard. Manufactured by reacting dry ethylene with sulfur monochloride under controlled conditions. Undistilled sulfur mustard contains 20–30% impurities, for which reason it does not store as well as HD. Also, as it decomposes, it increases in vapor pressure, making the munition it is contained in likely to split, especially along a seam, thus releasing the agent to the atmosphere

HD – Codenamed Pyro by the British, and Distilled Mustard by the US. Distilled sulfur mustard (bis(2-chloroethyl) sulfide); approximately 96% pure. The term "mustard gas" usually refers to this variety of sulfur mustard. A much used path of synthesis was based upon the reaction of thiodiglycol with hydrochloric acid.
HT – Codenamed Runcol by the British, and Mustard T- mixture by the US. A mixture of 60% sulfur mustard (HD) and 40% T (bis[2-(2-chloroethylthio)ethyl] ether), a related vesicant with lower freezing point, lower volatility and similar vesicant characteristics).
HL – A blend of distilled mustard (HD) and Lewisite (L), originally intended for use in winter conditions due to its lower freezing point compared to the pure substances. The Lewisite component of HL was used as a form of antifreeze.
HQ – A blend of distilled mustard (HD) and sesquimustard (Q) (Gates and Moore 1946).

The complete list of effective sulfur mustard agents commonly stock-piled is as follows:

1,2-Bis(2-chloroethylthio) ethane (aka Sesquimustard; Q)
1,3-Bis(2-chloroethylthio)-n-propane
1,4-Bis(2-chloroethylthio)-n-butane
1,5-Bis(2-chloroethylthio)-n-pentane
2-Chloroethylchloromethylsulfide
Bis(2-chloroethyl) sulfide (HD)
Bis(2-chloroethylthio) methane
Bis(2-chloroethylthiomethyl) ether
Bis(2-chloroethylthioethyl) ether (O Mustard)

Chemistry

Mustard gas is the organic compound described with the formula (ClCH₂CH₂)₂S. It has several names (see table). In the Depretz method, mustard gas is synthesized by treating sulfur dichloride with ethylene:

SCl₂ + 2 C₂H₄ → (ClCH₂CH₂)₂S

In the Meyer method, thiodiglycol is produced from chloroethanol and potassium sulfide and chlorinated with phosphorus trichloride:

3(HO-CH₂CH₂)₂S + 2PCl₃ → 3(Cl-CH₂CH₂)₂S + 2P(OH)₃

In the Meyer-Clarke method, concentrated hydrochloric acid (HCl) instead of PCl₃ is used as the chlorinating agent:

(HO-CH₂CH₂)₂S + 2HCl → (Cl-CH₂CH₂)₂S + 2H₂O

Thionyl chloride and phosgene have also been used as chlorinating agents.

Although the compound is commonly known as "mustard gas", it is a viscous liquid at normal temperatures. The pure compound has a melting point of 14°C (57°F) and decomposes before boiling at 218 °C (423 °F).

The compound readily eliminates chloride ion by intramolecular nucleophilic substitution to form a cyclic sulfonium ion. This very reactive intermediate tends to bond to the guanine nucleotide in DNA strands, which is particularly detrimental to cellular health. This alkylation leads to either cellular death or cancer. Mustard gas is not very soluble in water but is very soluble in fat, contributing to its rapid absorption into the skin.

In the wider sense, compounds with the structural element BCH₂CH₂X, where B is any leaving group and X is a Lewis base are known as mustards. Such compounds can form cyclic "onium" ions (sulfonium, ammoniums, etc.) that are good alkylating agents. Examples are bis(2-chloroethyl)ether, the (2-haloethyl)amines (nitrogen mustards), and sulfur sesquimustard, which has two α-chloroethyl thioether groups (ClH₂C-CH₂-S-) connected by an ethylene (-CH₂CH₂-) group. These compounds have a similar ability to alkylate DNA, but their physical properties, e.g. melting point, vary.

History

Development

Mustard gas was possibly developed as early as 1822 by M. Depretz (1798–1863). Depretz described the reaction of sulfur dichloride and ethene but never made mention of any irritating properties of the reaction product which makes the claim doubtful. In 1854, another French chemist Alfred Riche (1829–1908) repeated the procedure but again did not describe any adverse physiological properties. In 1886, chemist Albert Niemann, known as a pioneer in cocaine chemistry, repeated the reaction but this time blister forming properties were recorded. In 1860, Frederick Guthrie synthesised and characterized the compound, and he also noted its irritating properties especially in tasting. In 1886, Viktor Meyer published a paper describing a synthesis which produced good yields. He reacted 2-chloroethanol with aqueous potassium sulfide and treated the resulting thiodiglycol with phosphorus trichloride. The purity of this compound was much higher and the adverse health effects on exposure consequently much more severe. These symptoms presented themselves in an assistant, and in order to rule out that the assistant was suffering from a mental illness (faking the symptoms) Meyer had the compound tested on rabbits, which consequently died. In 1913, English chemist Hans T. Clarke (of Eschweiler-Clarke fame) replaced phosphorus trichloride with hydrochloric acid in Meyers recipe while working with Emil Fischer in Berlin. Clarke was hospitalized for 2 months for burns after a flask broke, and according to him Fisher's subsequent report on this incident to the German Chemical Society set Germany on the chemical weapons track. Germany in World War I relied on the Meyer-Clarke method with a 2-chloroethanol infrastructure already in place in the dye industry of that time.

Use

Mustard gas was first used effectively in World War I by the German army against British soldiers near Ypres in July 1917 and later also against the French Second Army. The name Yperite comes from its usage by the German army near the city of Ypres. The Allies did not use mustard until November 1917 at Cambrai, after they captured a large stock of German mustard-filled shells. It took the British over a year to develop their own mustard gas weapon (their only option was the Despretz–Niemann–Guthrie process), first using it in September 1918 during the breaking of the Hindenburg Line.

Mustard gas was dispersed as an aerosol in a mixture with other chemicals, giving it a yellow-brown color and a distinctive odor. Mustard gas has also been dispersed in such munitions as aerial bombs, land mines, mortar rounds, artillery shells, and rockets. Mustard gas was lethal in only about 1% of cases; its effectiveness was as an incapacitating agent. Countermeasures against the gas were relatively ineffective, since a soldier wearing a gas mask was not protected against absorbing it through the skin.

Furthermore, mustard gas was a persistent agent which would remain in the environment for days and continue to cause sickness. If mustard gas contaminated a soldier's clothing and equipment, then other soldiers he came into contact with would also be poisoned. Towards the end of the war it was even used in high concentrations as an area-denial weapon, which often forced soldiers to abandon heavily contaminated positions.

Since then, mustard gas has also been reportedly used in several wars, often where those it is used against cannot retaliate:

United Kingdom against the Red Army in 1919;
Spain against Rif insurgents in Morocco in 1921-1927;
Italy in Libya in 1930;
Soviet Union in Xinjiang, China in 1934 and 1936-1937;
Italy against Abyssinia (now Ethiopia) from 1935 to 1940;
Poland against Germany in 1939 during an isolated incident, British product;
Germany against Poland and the Soviet Union in a few erroneous uses during the Second World War;
Japan against China in 1937-1945;
Egypt against North Yemen in 1963-1967;
Iraq against Iran in 1983-1988;
Possibly Sudan against insurgents in the civil war, in 1995 and 1997

In 1943, during the Second World War, a U.S. stockpile exploded aboard a supply ship that was bombed in an air raid in the harbor of Bari, Italy, exposing and killing thousands of civilians and 628 Allied troops. The deaths and incident were classified Top Secret for 55 years and labeled as a mystery illness. It was noted by the U.S. Army's medical workers that the white cell counts of exposed soldiers were reduced, and mustard gas was investigated as a therapy for Hodgkin's lymphoma, a form of cancer. Study of the use of similar chemicals as agents for the treatment of cancers led to the discovery of mustine, and the birth of anticancer chemotherapy.

The use of poison gas, including mustard gas, during warfare, a practice known as chemical warfare, was prohibited by the Geneva Protocol of 1925 and the subsequent Chemical Weapons Convention of 1993, which also prohibits the development, production and stockpiling of such weapons.

Disposal

Most of the mustard gas found in Germany after World War II was dumped into the Baltic Sea. Between 1966 and 2002, fishermen have found around 700 chemical weapons outside Bornholm, most of which were mustard gas bombs. When mustard gas is exposed to seawater, it forms a tar-like gel and maintains its lethality for at least five years. It is possible to mistake a piece of polymerised mustard gas for ambergris, which can lead to severe health problems. Shells containing mustard gas and other toxic ammunition from World War I (as well as conventional explosives) can still occasionally be found in France and Belgium; they used to be disposed of by explosion at sea, but current environmental regulations prohibit this and so the French government is building an automated factory to dispose of the backlog of shells.

In 1972, the United States Congress banned the practice of disposing chemical weapons into the ocean. However, 64 million pounds of nerve and mustard agents had already been dumped into the ocean waters off the United States by the U.S. Army. According to a 1998 report created by William Brankowitz, a deputy project manager in the U.S. Army Chemical Materials Agency, the Army created at least 26 chemical weapons dump sites in the ocean off at least 11 states on both the west and east coasts. Additionally because of poor records, they currently only know the rough whereabouts of half of them.

A significant portion of the stockpile of mustard agent in the United States was stored at the Edgewood Area of Aberdeen Proving Ground in Maryland. Approximately 1,621 tons of mustard agent was stored in one-ton (900 kg) containers on the base under heavy guard. A disposal plant built on site neutralized the last of this stockpile in February 2005. This stockpile had priority because of the potential for quick reduction of risk to the community. The closest schools were fitted with overpressurization units to protect the students and staff in the event of a catastrophic explosion and fire at the site. These projects, as well as planning, equipment, and training assistance, were provided to the surrounding community as a part of the Chemical Stockpile Emergency Preparedness Program (CSEPP), a joint US Army and Federal Emergency Management Agency program Unexploded shells containing mustard agent and other chemical agents are still present in several test ranges in proximity to Edgewood area schools, but the smaller amounts (4–14 pounds; 2–6 kg) present considerably less risk. They are being systematically detected and excavated for disposal. There are several other sites in the United States where the remaining U.S. stockpiles of chemical agents are awaiting destruction in compliance with international chemical weapons treaties; the largest mustard agent stockpile, approximately 6,196 tons, is stored at the Deseret Chemical Depot in Utah. Destruction of this stockpile began in 2006. U.S. mustard agent and other chemical agent storage is managed by the US Army's Chemical Materials Agency The Chemical Materials Agency (CMA) manages disposal operations at five of the remaining seven stockpile sites, located in Alabama, Arkansas, Indiana, Utah, and Oregon; disposal projects at the other two sites, located in Kentucky and Colorado, are managed by the Program Manager Assembled Chemical Weapons Alternatives (ACWA)

Physiological effects

Mustard gas has extremely powerful vesicant effects on its victims. Additionally, it is strongly mutagenic and carcinogenic, due to its alkylating properties. It is also lipophilic. Because people exposed to mustard gas rarely suffer immediate symptoms, and mustard-contaminated areas may appear completely normal, victims can unknowingly receive high dosages. However, within 6 to 24 hours of exposure to mustard agent, victims experience intense itching and skin irritation which gradually turns into large blisters filled with yellow fluid wherever the mustard agent contacted the skin. These are severe chemical burns which are slow to heal. If the victim's eyes were exposed then they become sore, starting with conjunctivitis, after which the eyelids swell, resulting in temporary blindness. According to the Medical Management of Chemical Casualties handbook, there have been experimental cases in humans where the patient has suffered miosis, or pinpointing of pupils, as a result of the cholinomimetic activity of mustard. At very high concentrations, if inhaled, mustard agent causes bleeding and blistering within the respiratory system, damaging mucous membranes and causing pulmonary edema. Severe mustard gas burns (i.e. where more than 50% of the victim's skin has been burned) are often fatal. Mild or moderate exposure to mustard agent is unlikely to kill, though victims invariably require lengthy periods of medical treatment and convalescence before recovery is complete. The mutagenic and carcinogenic effects of mustard agent mean that victims who recover from mustard gas burns have an increased risk of developing cancer in later life.

Skin damage can be reduced if povidone iodine in a base of glycofurol is rapidly applied, but since mustard agent initially has no symptoms, exposure is usually not recognised until skin irritation begins - at which point it is too late for countermeasures. The vesicant property of mustard gas can be neutralised by oxidation or chlorination; household bleach (sodium hypochlorite) or decontamination solution "DS2" (2% NaOH, 70% diethylenetriamine, 28% ethylene glycol monomethyl ether) can be used.