|Molar mass||159.08 g mol−1|
|Appearance||Colorless if pure.
Normally ranges from
pale yellow to dark brown.
Slight garlic or horseradish type odour
|Density||1.27 g/ml, liquid|
217 °C (decomposes)
|Solubility in water||Negligible|
|Main hazards||Very toxic (T+)
Dangerous for the environment (N)
|Flash point||105 °C|
|Related compounds||Nitrogen mustard|
(what is this?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
The sulfur mustards, of which mustard gas (1,5-dichloro-3-thiapentane) is a member, are a class of related cytotoxic, vesicant chemical warfare agents with the ability to form large blisters on exposed skin. Pure sulfur mustards are colorless, viscous liquids at room temperature. However, 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 name. Mustard gas was originally assigned the name LOST, after 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 (although this is somewhat incorrect, as nitrogen mustard is sometimes used in chemotherapy treatment of lung cancer and Hodgkins lymphoma.)
Mustard agents can be deployed on the battlefield via spraying from aircraft, or more typically by means of air-dropped bombs or artillery shells.
In the Meyer-Clarke method, concentrated hydrochloric acid (HCl) instead of PCl3 is used as the chlorinating agent:
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 (424.4 ° F).
The compound readily eliminates a 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 can lead to cellular death and 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 X is any leaving group and B 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.
Mustard gas has extremely powerful vesicant (skin irritating/blistering) 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 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 chemical burns and they are very debilitating. 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. Depending on the level of contamination, mustard gas burns can vary between first and second degree burns, though they can also be every bit as severe, disfiguring and dangerous as third degree burns. Severe mustard gas burns (i.e. where more than 50% of the victim's skin has been burned) are often fatal, with death occurring after some days or even weeks have passed. 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. After initial decontamination of the victim's wounds is complete, medical treatment is similar to that required by any conventional burn. The amount of pain and discomfort suffered by the victim is comparable, too. Mustard gas burns heal slowly and, as with other types of burn, there is a risk of sepsis caused by pathogens such as Staphylococcus aureus and Pseudomonas aeruginosa.
A British nurse treating soldiers with mustard gas burns during World War I commented:
They cannot be bandaged or touched. We cover them with a tent of propped-up sheets. Gas burns must be agonizing because usually the other cases do not complain even with the worst wounds but gas cases are invariably beyond endurance and they cannot help crying out.
In its history, various types and mixtures of sulfur mustard have been employed. These include:
The complete list of effective sulfur mustard agents commonly stock-piled is as follows:
Mustard gas was possibly developed as early as 1822 by M. Depretz (1798–1863). Depretz described the reaction of sulfur dichloride and ethylene 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 Thacher Clarke (of Eschweiler-Clarke fame) replaced phosphorus trichloride with hydrochloric acid in Meyer's recipe while working with Emil Fischer in Berlin. Clarke was hospitalized for 2 months for burns after a flask broke, and according to him Fischer'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.
Mustard gas was first used effectively in World War I by the German army against British soldiers near Ypres in 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 the gas 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 colour 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:
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 at least 83 Allied troops, and injuring nearly 600 more. The deaths and incident were partially classified for many years. 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.
From 1943 to 1944, mustard gas experiments were performed on Australian Army volunteers in tropical Queensland by British and U.S. Army experimenters, resulting in severe injuries. One test site, Brook Island, was chosen to simulate Japanese-held Pacific islands.
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.
Most of the sulfur mustard found in Germany after World War II was dumped into the Baltic Sea. Between 1966 and 2002, fishermen have found around 700 chemical weapons in the Bornholm region, most of which contained sulfur mustard. One of the more frequently dumped weapons was the "Sprühbüchse 37" (SprüBü37, Spray Can 37, 1937 being the year of its fielding with the German Army). These weapons contain sulfur mustard mixed with a thickener, which renders it a tarlike viscosity. When the content of the SprüBü37 comes in contact with water, only the sulfur mustard in the outer layers of the lumps of viscous mustard hydrolyses, leaving amber-coloured residues which still contain most of the active sulfur mustard. On mechanically breaking these lumps, e.g. with a fishing net's drag board or with the hands, the enclosed sulfur mustard is still as active as it had been at the time the weapon was dumped. These lumps, when washed ashore, can be mistaken for amber, which can lead to severe health problems. Shells containing sulfur mustard 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 (Operation CHASE, Operation Geranium, etc.). 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 U.S. Army Element, Assembled Chemical Weapons Alternatives (ACWA).
In 2008 a number of mustard gas bombs (empty) were recovered in an excavation at the Marrangaroo Army Base west of Sydney, Australia. In 2009, a mining survey near Chinchilla, Queensland uncovered 144 105mm Howitzer shells, some containing Mustard H, buried by the US Army during World War II.
Urinary concentrations of the thiodiglycol hydrolysis products of sulfur mustard have been used to confirm a diagnosis of chemical poisoning in hospitalized victims. The presence in urine of 1,1'-sulfonylbismethylthioethane (SBMTE), a conjugation product with glutathione, is considered a more specific marker, since this metabolite is not found in specimens from unexposed persons. Intact sulfur mustard was detected in postmortem fluids and tissues of a man who died one week post-exposure.