Methyldichloroarsine

Structure, synthesis, reactivity

Focusing on the arsenic center, the molecule geometry is trigonal 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:

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:

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

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. 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.

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

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. 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.