Arsenic trichloride is an inorganic compound

Inorganic compound

Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 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 compound

Organoarsenic compound

Organoarsenic chemistry is the chemistry of compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticides, herbicides, and fungicides. In general these applications are declining in...

s.

Structure

AsCl₃ is a pyramidal molecule with C_3v symmetry

Symmetry group

The symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...

. The As-Cl bond is 2.161 Å and the angle Cl-As-Cl is 98°25’±30. AsCl₃ has four normal modes of vibration: ν1(A₁) 416, ν2(A₁) 192, ν3 393, and ν4(E) 152 cm⁻¹.
Arsenic trichloride contains predominantly covalent bond

Covalent bond

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

s, which explains its low melting point.

Synthesis

This colourless liquid is prepared by treatment of arsenic(III) oxide

Arsenic trioxide

Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...

 with hydrogen chloride

Hydrogen chloride

The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

 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.

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:

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

Reactions

Hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 with water gives arsenous acid

Arsenous acid

Arsenous acid, also known as arsenious acid, is the inorganic compound with the formula H3AsO3. It is known to occur in aqueous solutions, but it has not been isolated as a pure material, although this fact does not detract from the significance of As3.-Properties:As3 is a pyramidal molecule...

 and hydrochloric acid

Hydrochloric acid

Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

:

AsCl₃ + 3 H₂O → As(OH)₃ + 3 HCl

Although AsCl₃ is less moisture sensitive than PCl₃, it still fumes in moist air.

AsCl₃ undergoes redistribution upon treatment with As₂O₃ to give the inorganic polymer

Inorganic polymer

Inorganic polymers are polymers with a skeletal structure that does not include carbon atoms. Polymers containing inorganic and organic components are named hybrid polymers. One of the best known examples is polydimethylsiloxane, otherwise known commonly as silicone rubber...

 AsOCl. With chloride sources, AsCl₃, forms salts containing the anion [AsCl₄]^–. Reaction with potassium bromide

Potassium bromide

Potassium bromide is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the United States. Its action is due to the bromide ion...

 and potassium iodide give arsenic tribromide

Arsenic tribromide

Arsenic tribromide is the chemical compound with the formula AsBr3. This pyramidal molecule is the only known binary arsenic bromide. It is formed by the direct reaction of bromine and arsenic. AsBr3 is noteworthy for its very high refractive index of approximately 2.3...

 and arsenic triiodide

Arsenic triiodide

Arsenic triiodide is the inorganic compound with the formula AsI3. It is a dark red solid that readily sublimes. It is a pyramidial molecule that is useful for preparing organoarsenic compounds.-Preparation:...

, respectively.

AsCl₃ is useful in organoarsenic chemistry, for example triphenylarsine

Triphenylarsine

Triphenylarsine is the chemical compound with the formula As3. This organoarsenic compound, often abbreviated AsPh3, is a colorless crystalline solid that is used as a ligand and a reagent in coordination chemistry and organic synthesis...

is derived from AsCl₃:

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.