Arsenous acid


Arsenous 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 As3.

Properties

As3 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. In contrast, the nominally related phosphorous acid H3PO3 adopts the structure HPO2. The structural analogue of arsenous acid 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 HAsO2, which is called arsonic acid. It has not been isolated or well-characterized.
exists in as the dihydroxy tautomer in contrast to arsenous acid.

Synthesis

The preparation of As3 involves a slow hydrolysis of arsenic trioxide in water. Addition of base converts arsenous acid to the arsenite ions −, 2−, and 3−.

Reactions

With its first pKa being 9.2, As3 is a weak acid. 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.
Reaction of arsenous acid with methyl iodide gives methylarsonic acid. This historically significant conversion is the Meyer reaction:
Alkylation occurs at As, and the oxidation state of As increases from III to V.

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.