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Methyldichloroarsine

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Methyldichloroarsine

Methyldichloroarsine
Structural formula Space-filling model
Identifiers
Abbreviations MD

MDA
MDCA

CAS number  YesY
PubChem
ChemSpider  YesY
MeSH
Jmol-3D images Image 1
Properties
Molecular formula 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)
Solubility in water reacts
Hazards
Main hazards Highly toxic, Irritant
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY   YesY/N?)

Methyldichloroarsine, sometimes abbreviated "MD", is an vesicant that has been used in chemical warfare.[1]

History

German chemists weaponized methyldichloroarsine during [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]

See also

References

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