Arsenic trichloride is an inorganic compound with the formula AsCl3, 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.Sabina C. Grund, Kunibert Hanusch, Hans Uwe Wolf "Arsenic and Arsenic Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, VCH-Wiley, 2008, Weinheim.
Structure AsCl3 is a pyramidal molecule with C3v symmetry. The As-Cl bond is 2.161 Å and the angle Cl-As-Cl is 98° 25'±30.P. Kisliuk; C. H. Townes. "The Microwave Spectra and Molecular Structure of Phosphorus and Arsenic Trichloride". J. Chem. Phys. 1950, 18.Jean Galy; Renee Enjalbertl Pierre Lecante; Andrzej Burian "AsCl3: From the crystalline to the liquid state. XRD (176< T (K) < 250) and WAXS (295K) studies" Inorg. Chem 2002, volume 41, pp. 693–698. AsCl3 has four normal modes of vibration: ν1(A1) 416, ν2(A1) 192, ν3 393, and ν4(E) 152 cm−1.Klapoetke, Thomas M. "The vibrational spectrum of arsenic trichloride" Main Group Metal Chemistry 1997, volume 20, pp. 81–83.
Synthesis This compound is prepared by treatment of arsenic(III) oxide with hydrogen chloride followed by distillation: As2O3 + 6 HCl → 2 AsCl3 + 3 H2O
It can also be prepared by chlorination of arsenic metal at 80–85 °C: 2 As + 3 Cl2 → 2 AsCl3 Arsenic trichloride can be prepared by the reaction of arsenic oxide and sulfur monochloride. This method requires simple apparatus and proceeds efficiently:R. C. Smith, "Manufacture of Arsenic trichloride" The Journal of Industrial and Engineering Chemistry 1919, volume 11, pp. 109–110. 2 As2O3 + 6 S2Cl2 → 4 AsCl3 + 3 SO2 + 9 S A convenient laboratory method is refluxing arsenic(III) oxide with thionyl chloride: 2 As2O3 + 3 SOCl2 → 2 AsCl3 + 3 SO2 Arsenic trichloride can also be prepared by the reaction of hydrochloric acid and arsenic(III) sulfide. As2S3 + 6 HCl → 2 AsCl3 + 3 H2S
Reactions Hydrolysis gives arsenous acid and hydrochloric acid: AsCl3 + 3 H2O → As(OH)3 + 3 HCl Although AsCl3 is less moisture sensitive than PCl3, it still fumes in moist air.Holleman, A. F.; Wiberg, E. Inorganic Chemistry Academic Press: San Diego, 2001. .
AsCl3 undergoes redistribution upon treatment with As2O3 to give the inorganic polymer AsOCl. With chloride sources, AsCl3 also forms salts containing the anion [AsCl4]−. Reaction with potassium bromide and potassium iodide give arsenic tribromide and arsenic triiodide, respectively.
AsCl3 is useful in organoarsenic chemistry, for example triphenylarsine is derived from AsCl3:. Describes the preparation of As(C6H5)3. AsCl3 + 6 Na + C6H5Cl → As(C6H5)3 + 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 AsCl3 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