Arsenic trichloride

Arsenic trichloride
Other names Arsenic(III) chloride, Arsenous trichloride, Butter of arsenic, de Valagin's solution
Identifiers
CAS number	7784-34-1 Y
PubChem	24570
ChemSpider	22974 Y
RTECS number	CG1750000
Jmol-3D images	Image 1
SMILES Cl[As](Cl)Cl
InChI InChI=1S/AsCl3/c2-1(3)4 Y Key: OEYOHULQRFXULB-UHFFFAOYSA-N Y InChI=1/AsCl3/c2-1(3)4 Key: OEYOHULQRFXULB-UHFFFAOYAG
Properties
Molecular formula	AsCl3
Molar mass	181.28 g/mol
Appearance	colourless liquid
Density	2.163 g/cm3, liquid
Melting point	-16.2 °C, 257 K, 3 °F
Boiling point	130.2 °C, 403 K, 266 °F
Solubility in water	decomposes
Solubility	soluble in alcohol, ether, HCl, HBr
Refractive index (nD)	1.6006
Viscosity	9.77 x 10-6 Pa s
Hazards
EU classification	Very Toxic (T+) Dangerous for the environment (N)
R-phrases	R23/25, R50/53
S-phrases	(S1/2), S20/21, S28, S45, S60, S61
NFPA 704	0 4 0
LD50	48 mg/kg
Related compounds
Other anions	Arsenic trioxide
Other cations	Antimony trichloride
Y trichloride (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.^[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] Arsenic trichloride contains predominantly covalent bonds, which explains its low melting point.

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 AsCl₃

Arsenic trichloride can also 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 S

Reactions

Hydrolysis with water 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.^[6]

AsCl₃ undergoes redistribution upon treatment with As₂O₃ to give the inorganic polymer AsOCl. With chloride sources, AsCl₃, 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:^[7]

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

Safety

Arsenic compounds are highly toxic, and AsCl₃ especially so because of its volatility and solubility.

References

1. ^ ^{a b} 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
2. ^ P.Kisliuk; C.H.Townes. The Microwave Spectra and Molecular Structure of Phosphorus and Arsenic Trichloride. J. Chem. Physics 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.doi:10.1021/ic0102788
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. doi:10.1021/ie50110a009
6. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
7. ^ Shriner, R. L.; Wolf, C. N. (1963), "Tetraphenylarsonium Chloride Hydrochloride", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv4p0910 ; Coll. Vol. 4: 910 . Describes the preparation of As(C₆H₅)₃.