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Arsenic trichloride

Arsenic trichloride is an inorganic compound with the formula AsCl<sub>3</sub>, also known as arsenous chloride or butter of arsenic. This poisonous oil is colourless, although impure samples may appear yellow. It is an intermediate in the manufacture of organoarsenic compounds.

Structure

AsCl<sub>3</sub> is a pyramidal molecule with C<sub>3v</sub> symmetry. The As-Cl bond is 2.161 Å and the angle Cl-As-Cl is 98° 25'±30. AsCl<sub>3</sub> has four normal modes of vibration: ν1(A<sub>1</sub>) 416, ν2(A<sub>1</sub>) 192, ν3 393, and ν4(E) 152&nbsp;cm<sup>−1</sup>.

Synthesis

This compound is prepared by treatment of arsenic(III) oxide with hydrogen chloride followed by distillation:

As<sub>2</sub>O<sub>3</sub> + 6 HCl → 2 AsCl<sub>3</sub> + 3 H<sub>2</sub>O

It can also be prepared by chlorination of arsenic metal at 80–85&nbsp;°C:

2 As + 3 Cl<sub>2</sub> → 2 AsCl<sub>3</sub>

Arsenic trichloride can be prepared by the reaction of arsenic oxide and sulfur monochloride. This method requires simple apparatus and proceeds efficiently:

2 As<sub>2</sub>O<sub>3</sub> + 6 S<sub>2</sub>Cl<sub>2</sub> → 4 AsCl<sub>3</sub> + 3 SO<sub>2</sub> + 9 S

A convenient laboratory method is refluxing arsenic(III) oxide with thionyl chloride:

2 As<sub>2</sub>O<sub>3</sub> + 3 SOCl<sub>2</sub> → 2 AsCl<sub>3</sub> + 3 SO<sub>2</sub>

Arsenic trichloride can also be prepared by the reaction of hydrochloric acid and arsenic(III) sulfide.

As<sub>2</sub>S<sub>3</sub> + 6 HCl → 2 AsCl<sub>3</sub> + 3 H<sub>2</sub>S

Reactions

Hydrolysis gives arsenous acid and hydrochloric acid:

AsCl<sub>3</sub> + 3 H<sub>2</sub>O → As(OH)<sub>3</sub> + 3 HCl

Although AsCl<sub>3</sub> is less moisture sensitive than PCl<sub>3</sub>, it still fumes in moist air.

AsCl<sub>3</sub> undergoes redistribution upon treatment with As<sub>2</sub>O<sub>3</sub> to give the inorganic polymer AsOCl. With chloride sources, AsCl<sub>3</sub> also forms salts containing the anion [AsCl<sub>4</sub>]<sup>−</sup>. Reaction with potassium bromide and potassium iodide give arsenic tribromide and arsenic triiodide, respectively.

AsCl<sub>3</sub> is useful in organoarsenic chemistry, for example triphenylarsine is derived from AsCl<sub>3</sub>:

AsCl<sub>3</sub> + 6 Na + C<sub>6</sub>H<sub>5</sub>Cl → As(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub> + 6 NaCl

The chemical weapons called Lewisites are prepared by the addition of arsenic trichloride to acetylene:

Safety

Inorganic arsenic compounds are highly toxic, and AsCl<sub>3</sub> especially so because of its volatility and solubility (in water).

A mixture of 50% hydrocyanic acid, 30% arsenic trichloride, 15% stannic chloride and 5% chloroform called Vincennite was used as chemical weapon by French forces in the World War I.

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.

References