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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.
Case Studies in Environmental Medicine - Arsenic Toxicity
{{DEFAULTSORT:Arsenous Acid Oxoacids Arsenites Arsenic(III) compounds
Properties
As(OH)3 is a pyramidal molecule consisting of threehydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
groups bonded to arsenic. The 1H NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry. In contrast, the nominally related phosphorous acid
Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...
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.
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 slowhydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
of arsenic trioxide
Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, it ...
in water. Addition of base converts arsenous acid to the arsenite ions sO(OH)2sup>−, sO2(OH)sup>2−, and sO3sup>3−.
Reactions
With its first p''K''a being 9.2, As(OH)3 is a weak acid. Reactions attributed to aqueousarsenic trioxide
Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, it ...
are due to arsenous acid and its conjugate bases.
Like arsenic trioxide, arsenous acid is sometimes amphoteric
In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used.
One type of amphoteric species are amphipro ...
. 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 methyl iodide gives methylarsonic acid. This historically significant conversion is the Meyer reaction:
:As(OH)3 + CH3I + NaOH ⇌ CH3AsO(OH)2 + NaI + H2O
Alkylation occurs at arsenic, and the oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
of arsenic increases from +3 to +5.
Toxicology
Arsenic-containing compounds are highly toxic andcarcinogenic
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
. The anhydride
An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...
form of arsenous acid, arsenic trioxide
Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, it ...
, is used as a herbicide, pesticide, and rodenticide
Rodenticides are chemicals made and sold for the purpose of killing rodents. While commonly referred to as "rat poison", rodenticides are also used to kill mice, squirrels, woodchucks, chipmunks, porcupines, nutria, beavers, and voles. Despi ...
.
References
External links
Case Studies in Environmental Medicine - Arsenic Toxicity
{{DEFAULTSORT:Arsenous Acid Oxoacids Arsenites Arsenic(III) compounds