Methyldichloroarsine, sometimes abbreviated "MD" and also known as methyl Dick, is an organoarsenic compound with the formula CH₃AsCl₂. This colourless volatile liquid is a highly toxic

vesicant A blister agent (or vesicant), is a chemical compound that causes severe skin, eye and mucosal pain and irritation. They are named for their ability to cause severe chemical burns, resulting in painful water blisters on the bodies of those affec ...

that has been used in

chemical warfare Chemical warfare (CW) involves using the toxic properties of chemical substances as weapons. This type of warfare is distinct from nuclear warfare, biological warfare and radiological warfare, which together make up CBRN, the military acronym ...

History

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

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 Methylmagnesium chloride is an organometallic compound with the general formula CH3MgCl. This highly flammable, colorless, and moisture sensitive material is the simplest Grignard reagent and is commercially available, usually as a solution in te ...

and arsenic trichloride: :AsCl₃ + CH₃MgCl → CH₃AsCl₂ + MgCl₂ Typically such syntheses are conducted in ether or

THF Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...

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 In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...

sodium arsenite Sodium arsenite usually refers to the inorganic compound with the formula NaAsO2. Also called sodium ''meta''-arsenite, it is the sodium salt of arsenous acid. Sodium ''ortho''-arsenite is Na3AsO3.Greenwood, N. N.; & Earnshaw, A. (1997). Chemistr ...

: :2 Na₃AsO₃ + (CH₃O)₂SO₂ → 2 CH₃AsO(ONa)₂ + Na₂SO₄, followed by reduction of the disodium monomethylarsonate with

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...

: :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 H₃Assub>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 Irritation, in biology and physiology, is a state of inflammation or painful reaction to allergy or cell-lining damage. A stimulus or agent which induces the state of irritation is an irritant. Irritants are typically thought of as chemical age ...

to the eyes and to the nose, although

blister A blister is a small pocket of body fluid (lymph, serum, plasma, blood, or pus) within the upper layers of the skin, usually caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection. Most blisters are filled wi ...

ing may be delayed for hours. Other symptoms include: dermal burns with vesicle formation;

blepharospasm Blepharospasm is any abnormal contraction of the orbicularis oculi muscle. The condition should be distinguished from the more common, and milder, involuntary quivering of an eyelid, known as myokymia, or fasciculation. In most cases, blepharosp ...

and photophobia.

Convulsion A convulsion is a medical condition where the body muscles contract and relax rapidly and repeatedly, resulting in uncontrolled shaking. Because epileptic seizures typically include convulsions, the term ''convulsion'' is sometimes used as a s ...

s, abdominal pain,

coughing A cough is a sudden expulsion of air through the large breathing passages that can help clear them of fluids, irritants, foreign particles and microbes. As a protective reflex, coughing can be repetitive with the cough reflex following three phas ...

, 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

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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 Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...

and caustic soda.

References