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Arsenic trichloride
Names
	Other names Arsenic(III) chloride, Arsenous trichloride, Arsine trichloride, Butter of arsenic, de Valagin's solution, Trichloroarsine
Identifiers
CAS Number	7784-34-1 ;
3D model (JSmol)	Interactive image;
ChemSpider	22974 ;
ECHA InfoCard	100.029.144
EC Number	232-059-5;
PubChem CID	24570;
RTECS number	CG1750000;
UNII	5XW39M1300;
UN number	1560
CompTox Dashboard (EPA)	DTXSID3042556 ;
	InChI InChI=1S/AsCl3/c2-1(3)4 Key: OEYOHULQRFXULB-UHFFFAOYSA-N ; InChI=1/AsCl3/c2-1(3)4 Key: OEYOHULQRFXULB-UHFFFAOYAG;
	SMILES Cl[As](Cl)Cl;
Properties
Chemical formula	AsCl3
Molar mass	181.28 g/mol
Appearance	colourless oily liquid
Density	2.163 g/cm3, liquid
Melting point	−16.2 °C (2.8 °F; 256.9 K)
Boiling point	130.2 °C (266.4 °F; 403.3 K)
Solubility in water	Hydrolyzes
Solubility	soluble in alcohol, ether, HCl, HBr, chloroform, CCl4
Magnetic susceptibility (χ)	-79.9·10−6 cm3/mol
Refractive index (nD)	1.6006
Viscosity	9.77 x 10−6 Pa s
Hazards
	Occupational safety and health (OHS/OSH):
Main hazards	Very toxic, carcinogen, corrosive, decomposes on contact with water releasing HCl
	GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H301, H310, H314, H331, H350, H410
Precautionary statements	P201, P202, P260, P261, P262, P264, P270, P271, P273, P280, P281, P301+P310, P301+P330+P331, P302+P350, P303+P361+P353, P304+P340, P305+P351+P338, P308+P313, P310, P311, P321, P322, P330, P361, P363, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)	4 0 1
	Lethal dose or concentration (LD, LC):
LD50 (median dose)	48 mg/kg
LCLo (lowest published)	100 mg/m3 (cat, 1 hr); 200 mg/m3 (cat, 20 min); 338 ppm (rat, 10 min)
	NIOSH (US health exposure limits):
PEL (Permissible)	[1910.1018] TWA 0.010 mg/m3
REL (Recommended)	Ca C 0.002 mg/m3 [15-minute]
IDLH (Immediate danger)	Ca [5 mg/m3 (as As)]
Related compounds
Other anions	Arsenic trioxide, Arsenic trifluoride
Other cations	Antimony trichloride
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

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.^[4]

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.^[5]^[6] AsCl₃ has four normal modes of vibration: ν1(A₁) 416, ν2(A₁) 192, ν3 393, and ν4(E) 152 cm⁻¹.^[7]

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.^[4]

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:^[8]

2 As₂O₃ + 6 S₂Cl₂ → 4 AsCl₃ + 3 SO₂ + 9 S

A convenient laboratory method is refluxing arsenic(III) oxide with thionyl chloride:^[9]

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.^[10]

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:^[11]

AsCl₃ + 6 Na + C₆H₅Cl → As(C₆H₅)₃ + 6 NaCl

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

AsCl₃ + C₂H₂ → ClCH=CHAsCl₂

Safety

Inorganic arsenic compounds are highly toxic,^[4] 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.^[12]

References

^ John Rumble (June 18, 2018). CRC Handbook of Chemistry and Physics (99th ed.). CRC Press. pp. 4–41. ISBN 978-1138561632.
^ ^a ^b ^c NIOSH Pocket Guide to Chemical Hazards. "#0038". National Institute for Occupational Safety and Health (NIOSH).
^ "Arsenic (inorganic compounds, as As)". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
^ ^a ^b ^c Sabina C. Grund, Kunibert Hanusch, Hans Uwe Wolf "Arsenic and Arsenic Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, VCH-Wiley, 2008, Weinheim.doi:10.1002/14356007.a03_113.pub2
^ 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.doi:10.1021/ic0102788
^ Klapoetke, Thomas M. "The vibrational spectrum of arsenic trichloride" Main Group Metal Chemistry 1997, volume 20, pp. 81–83.
^ R. C. Smith, "Manufacture of Arsenic trichloride" The Journal of Industrial and Engineering Chemistry 1919, volume 11, pp. 109–110. doi:10.1021/ie50110a009
^ Pandey, S. K.; Steiner, A.; Roesky, H. W. (1997). "Arsenic(III) chloride". Inorganic Syntheses. 31: 148-150. doi:10.1002/9780470132623.ch24.
^ Holleman, A. F.; Wiberg, E. Inorganic Chemistry Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
^ Shriner, R. L.; Wolf, C. N. (1963). "Tetraphenylarsonium Chloride Hydrochloride". Organic Syntheses; Collected Volumes, vol. 4, p. 910.. Describes the preparation of As(C₆H₅)₃.
^ 40 C.F.R.: Appendix A to Part 355—The List of Extremely Hazardous Substances and Their Threshold Planning Quantities (PDF) (Report) (July 1, 2008 ed.). Government Printing Office. Archived from the original (PDF) on February 25, 2012. Retrieved October 29, 2011.