Arsenous acid (or arsorous acid) is the

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula H₃AsO₃. It is known to occur in

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...

s, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)₃.

Properties

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

hydroxyl 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 hydroxy ...

groups bonded to arsenic. The ¹H

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

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) is the Compound (chemistry), compound described by the chemical formula, formula . It is diprotic (readily ionizes two protons), not triprotic as might be suggested by its formula. Phosphorous acid is an in ...

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 slow

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

arsenic trioxide Arsenic trioxide is an inorganic compound with the formula . As an industrial chemical, its major uses include the manufacture of wood preservatives, pesticides, and glass. It is sold under the brand name Trisenox among others when used as a m ...

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 aqueous

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. Etymology and terminology Amphoteric is d ...

. 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 Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one h ...

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 Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

of arsenic increases from +3 to +5.

Toxicology

Arsenic-containing compounds are highly toxic and

carcinogenic A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and Biological agent, biologic agent ...

. The

anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid. In organic chemistry, organic acid anhydrides contain the functional group . Organic acid anhydrides often form when one ...

form of arsenous acid,

, is used as a

herbicide Herbicides (, ), also commonly known as weed killers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page f ...

pesticide Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...

, 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, woodchucks, chipmunks, porcupines, nutria, beavers, and voles. Some rodenticides ...

References

External links

Case Studies in Environmental Medicine - Arsenic Toxicity
{{DEFAULTSORT:Arsenous Acid Pnictogen oxoacids Arsenites Arsenic(III) compounds