Arsenous acid

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Arsenous acid
Arsenous-acid-2D.svg
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Names
IUPAC name
Arsorous acid
Other names
Arsenious acid
Arsenic oxide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
DrugBank
PubChem CID
UNII
  • InChI=1S/AsH3O3/c2-1(3)4/h2-4H Yes check.svgY
    Key: GCPXMJHSNVMWNM-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/AsH3O3/c2-1(3)4/h2-4H
    Key: GCPXMJHSNVMWNM-UHFFFAOYAA
  • O[As](O)O
Properties
H3AsO3
Molar mass 125.94 g/mol
AppearanceOnly exists in aqueous solutions
Conjugate base Arsenite
-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).
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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]

Contents

Properties

Phosphorous acid exists as the dihydroxy tautomer in contrast to arsenous acid. H3PO3new.svg
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 pKa 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.

References

  1. 1 2 3 NIOSH Pocket Guide to Chemical Hazards. "#0038". National Institute for Occupational Safety and Health (NIOSH).
  2. ↑ Munoz-Hernandez, M.-A. (1994). "Arsenic: Inorganic Chemistry". In King, R. B. (ed.). Encyclopedia of Inorganic Chemistry. Chichester: John Wiley & Sons.
  3. ↑ Kolozsi, A.; Lakatos, A.; Galbács, G.; Madsen, A. Ø.; Larsen, E.; Gyurcsik, B. (2008). "A pH-Metric, UV, NMR, and X-ray Crystallographic Study on Arsenous Acid Reacting with Dithioerythritol" (PDF). Inorganic Chemistry. 47 (9): 3832–3840. doi:10.1021/ic7024439. PMID   18380458. Archived from the original (PDF) on 2012-04-25. Retrieved 2011-12-18.
  4. 1 2 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  5. ↑ Hernández-Molina, Rita; Sokolov, Maxim N.; Clausen, Maria; Clegg, William (2006). "Synthesis and Structure of Nickel-Containing Cuboidal Clusters Derived from [W3Se4(H2O)9]4+. Site-Differentiated Substitution at the Nickel Site in the Series [W3NiQ4(H2O)10]4+(Q = S, Se)". Inorganic Chemistry. 45 (26): 10567–10575. doi:10.1021/ic061146n. PMID   17173411.
  6. ↑ G. Meyer (1883). "Ueber einige anomale Reaktionen". Berichte der Deutschen Chemischen Gesellschaft. 13: 1439–1443. doi:10.1002/cber.188301601316.
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