Arsenic trichloride explained

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.^[1]

Structure

AsCl₃ is a pyramidal molecule with C_3v symmetry. The As-Cl bond is 2.161 Å and the angle Cl-As-Cl is 98° 25'±30.^{[2] [3]} AsCl₃ has four normal modes of vibration: ν1(A₁) 416, ν2(A₁) 192, ν3 393, and ν4(E) 152 cm⁻¹.^[4]

Synthesis

This colourless liquid 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 at 80–85 °C, but this method requires elemental arsenic.^[1]

2 As + 3 Cl₂ → 2 AsCl₃Arsenic trichloride can be prepared by the reaction of arsenic oxide and sulfur monochloride. This method requires simple apparatus and proceeds efficiently:^[5]

2 As₂O₃ + 6 S₂Cl₂ → 4 AsCl₃ + 3 SO₂ + 9 SA convenient laboratory method is refluxing arsenic(III) oxide with thionyl chloride:^[6]

2 As₂O₃ + 3 SOCl₂ → 2 AsCl₃ + 3 SO₂Arsenic 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 HClAlthough AsCl₃ is less moisture sensitive than PCl₃, it still fumes in moist air.^[7]

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:^[8]

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,^[1] and AsCl₃ especially so because of its volatility and solubility (in water).

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.^[9]

Notes and References

1. Sabina C. Grund, Kunibert Hanusch, Hans Uwe Wolf "Arsenic and Arsenic Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, VCH-Wiley, 2008, Weinheim.
2. P. Kisliuk; C. H. Townes. "The Microwave Spectra and Molecular Structure of Phosphorus and Arsenic Trichloride". J. Chem. Phys. 1950, 18.
3. 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.
4. Klapoetke, Thomas M. "The vibrational spectrum of arsenic trichloride" Main Group Metal Chemistry 1997, volume 20, pp. 81–83.
5. R. C. Smith, "Manufacture of Arsenic trichloride" The Journal of Industrial and Engineering Chemistry 1919, volume 11, pp. 109–110.
6. S. K.. Pandey . A. . Steiner . H. W. . Roesky . Arsenic(III) chloride . Inorganic Syntheses . 1997. 31. 148-150. 10.1002/9780470132623.ch24.
7. Holleman, A. F.; Wiberg, E. Inorganic Chemistry Academic Press: San Diego, 2001. .
8. . Describes the preparation of As(C₆H₅)₃.
9. . 40 C.F.R.: Appendix A to Part 355—The List of Extremely Hazardous Substances and Their Threshold Planning Quantities . July 1, 2008 . October 29, 2011 . https://web.archive.org/web/20120225051612/http://edocket.access.gpo.gov/cfr_2008/julqtr/pdf/40cfr355AppA.pdf . February 25, 2012 . dead .