Arsenous acid

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Arsenous acid
Structural formula
Ball-and-stick model
Names
IUPAC name
Arsorous acid
Other names
Arsenious acid
Arsenic oxide
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
DrugBank
UNII
  • InChI=1S/AsH3O3/c2-1(3)4/h2-4H checkY
    Key: GCPXMJHSNVMWNM-UHFFFAOYSA-N checkY
  • InChI=1/AsH3O3/c2-1(3)4/h2-4H
    Key: GCPXMJHSNVMWNM-UHFFFAOYAA
  • O[As](O)O
Properties
H3AsO3
Molar mass 125.94 g/mol
Appearance Only 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 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 As(OH)3.[2]

Properties

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

As(OH)3 is a pyramidal molecule consisting of three

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 is HAsO(OH)2, which is called arsonic acid. It has not been isolated or well-characterized.

Synthesis

The preparation of As(OH)3 involves a slow hydrolysis of arsenic trioxide in water. Addition of base converts arsenous acid to the arsenite ions [AsO(OH)2]−, [AsO2(OH)]2−, and [AsO3]3−.

Reactions

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 HCl ⇌ AsCl3 + 3 H2O
As(OH)3 + 3 HBr ⇌ AsBr3 + 3 H2O
As(OH)3 + 3 HI ⇌ AsI3 + 3 H2O

Reaction of arsenous acid with

Meyer reaction:[5]

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

anhydride form of arsenous acid, arsenic trioxide, is used as a herbicide, pesticide, and rodenticide
.

References

  1. ^ a b c 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. PMID 18380458. Archived from the original
    (PDF) on 2012-04-25. Retrieved 2011-12-18.
  4. ^
    ISBN 978-0-08-037941-8
    .
  5. doi:10.1002/cber.188301601316
    .

External links
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