Methyldichloroarsine

Methyldichloroarsine, sometimes abbreviated “MD”, is an organoarsenic compound with the formula CH3AsCl2. This colourless volatile liquid is a highly toxic vesicant that has been used in chemical warfare.[1]

Methyldichloroarsine
Structural formula
Space-filling model
Names
Preferred IUPAC name
Dichloromethylarsane
Systematic IUPAC name
Dichloro(methyl)arsane
Identifiers
3D model (JSmol)
Abbreviations MD

MDA
MDCA

ChemSpider
MeSH Methyldichloroarsine
Properties
CH3AsCl2
Molar mass 160.86 g·mol−1
Appearance Colorless liquid
Density 1.836 g/cm3
Melting point −55 °C (−67 °F; 218 K)
Boiling point 133 °C (271 °F; 406 K)
reacts
Hazards
Main hazards Highly toxic, Irritant
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

History

German chemists weaponized methyldichloroarsine during World War I, between 1917 and 1918. It was the first organoarsenic compound to be weaponized.[1]

Structure, synthesis, reactivity

Focusing on the arsenic center, the molecule is pyramidal with the Cl-As-Cl and C-As-Cl angles approaching 90° (see image). Virtually all related arsenic(III) compounds adopt similar structures.

Methyldichloroarsine is produced by the reaction of methylmagnesium chloride and arsenic trichloride:[2]

AsCl3 + CH3MgCl → CH3AsCl2 + MgCl2

Typically such syntheses are conducted in ether or THF solutions and typically the product is isolated by distillation. Use of larger amounts of the magnesium reagent affords greater amounts of dimethylchloroarsine ((CH3)2AsCl) and trimethylarsine ((CH3)3As).

In World War I, the German manufacturing method consisted of a three-step reaction beginning with methylation of sodium arsenite:

2 Na3AsO3 + (CH3O)2SO2 → 2 CH3AsO(ONa)2 + Na2SO4,

followed by reduction of the disodium monomethylarsonate with sulfur dioxide:

CH3AsO(ONa)2 + SO2 → CH3AsO + Na2SO4,

subsequently reacting the monomethylarsine oxide thus formed with hydrogen chloride to yield methyldichloroarsine:[3]

CH3AsO + 2 HCl → CH3AsCl2 + H2O

The As-Cl bonds in MD are susceptible toward nucleophilic attack. Reduction of MD with sodium metal affords the polymer [CH3As]n.

Use as a weapon

Methyldichloroarsine's only documented large-scale use is as a chemical weapon.

Symptoms of poisoning

Although some of its symptoms resemble those from poison ivy, other symptoms include irritation to the eyes and to the nose, although blistering may be delayed for hours.[4] Other symptoms include: dermal burns with vesicle formation; blepharospasm and photophobia. Convulsions, abdominal pain, coughing, and shortness of breath with damage to the respiratory system can be delayed for about three to five days; hemolysis can also occur.[1]

MD is not persistent, meaning that it will dissipate after a short time.[4] It is, however, still quite lethal. The LCt/50 for MD is about 3,000 mg/(min * m3).[4]

Protection

Besides avoiding situations in which it might be used, an activated charcoal filter and a protective mask can help protect against MD. It should, however, be noted that MD can penetrate rubber, so some masks and clothing are ineffective.[2] Other protective clothing, such as full body protection, are useful as well. Among the agents useful for decontamination of MD are bleach and caustic soda.[4]

gollark: Accurate orbital prediction is as far as I know basically a solved problem, GPS relies on it.
gollark: True, true.
gollark: You don't really need to *destroy* the incoming rod as much as deflect it a few degrees though, no?
gollark: They radiate tons of heat and you can perfectly predict their orbital path given previous knowledge of it.
gollark: Space things are on the whole not very stealthy anyway.

See also

References
  1. Fitzgerald, G. M.; Vollmer, T. (2006-06-19). "CBRNE - Vesicants, Organic Arsenicals: L, ED, MD, PD, HL". WebMD. Retrieved 2008-12-23.
  2. Ledgard, J. (2006). The Laboratory History of Chemical Warfare Agents. Lulu.com. p. 117. ISBN 9781411694323.
  3. Lohs, K. H. (1974). Synthetische Gifte (in German) (4th ed.). Berlin (East), GDR: Militärverlag der Deutschen Demokratischen Republik.
  4. Cashman, J. R. (2008). Emergency Response Handbook for Chemical and Biological Agents and Weapons (2nd ed.). CRC Press. p. 216. ISBN 9781420052664.
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