Arsenous acid

Arsenous acid (or arsorous 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 As(OH)₃.^[2]

Arsenous acid

Names
IUPAC name Arsorous acid
Other names Arsenious acid Arsenic oxide
Identifiers
CAS Number	13464-58-9 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:49900 Y
ChemSpider	530 Y
DrugBank	DB04456 Y
PubChem CID	545
UNII	935XD1L5K2 Y
CompTox Dashboard (EPA)	DTXSID7074828
InChI InChI=1S/AsH3O3/c2-1(3)4/h2-4H Y Key: GCPXMJHSNVMWNM-UHFFFAOYSA-N Y InChI=1/AsH3O3/c2-1(3)4/h2-4H Key: GCPXMJHSNVMWNM-UHFFFAOYAA
SMILES O[As](O)O
Properties
Chemical formula	H3AsO3
Molar mass	125.94 g/mol
Appearance	Only exists in aqueous solutions
Conjugate base	Arsenite
Magnetic susceptibility (χ)	-51.2·10−6 cm3/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Toxic, corrosive
NIOSH (US health exposure limits):
PEL (Permissible)	[1910.1018] TWA 0.010 mg/m3[1]
REL (Recommended)	Ca C 0.002 mg/m3 [15-minute][1]
IDLH (Immediate danger)	Ca [5 mg/m3 (as As)][1]
Related compounds
Related compounds	Arsenic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Properties

Phosphorous acid exists as the dihydroxy tautomer in contrast to arsenous acid.

As(OH)₃ is a pyramidal molecule consisting of three hydroxyl groups bonded to arsenic. The ¹H NMR spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry.^[3] In contrast, the nominally related phosphorous acid H₃PO₃ adopts the structure HPO(OH)₂. The structural analogue of arsenous acid (P(OH)₃) is a very minor equilibrium component of such solutions. The differing behaviors of the As and P compounds reflect a trend whereby high oxidation states are more stable for lighter members of main group elements than their heavier congeners.^[4] One tautomer of arsenous acid would have the formula HAsO(OH)₂, called arsonic acid, but it has not been characterized.

Coordination complexes of arsenous acid, i.e., complexes with the linkage M-As(OH)3, have been characterized by X-ray crystallography.^[5]

Synthesis

The preparation of As(OH)₃ involves a slow hydrolysis of arsenic trioxide in water.^{[citation needed]}

Reactions

Addition of base converts arsenous acid to the arsenite ions [AsO(OH)₂]⁻, [AsO₂(OH)]²⁻, and [AsO₃]³⁻. With its first pK_a being 9.2, As(OH)₃ is a weak acid.^[4] Reactions attributed to aqueous arsenic trioxide are due to arsenous acid and its conjugate bases.

Like arsenic trioxide, arsenous acid is sometimes amphoteric. For example, it reacts with hydrochloric, hydrobromic, and hydroiodic acids to produce arsenic trichloride, tribromide, and triiodide.

As(OH)₃ + 3 HX ⇌ AsX₃ + 3 H₂O (X = Cl, Br, I)

Reaction of arsenous acid with methyl iodide gives methylarsonic acid. This historically significant conversion is the Meyer reaction:^[6]

As(OH)₃ + CH₃I + NaOH ⇌ CH₃AsO(OH)₂ + NaI + H₂O

Alkylation occurs at arsenic, and the oxidation state of arsenic increases from +3 to +5.

Toxicology

Arsenic-containing compounds are highly toxic and carcinogenic. The anhydride form of arsenous acid, arsenic trioxide, is used as a herbicide, pesticide, and rodenticide.