Arsenous 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. 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. 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. ==Synthesis==

The preparation of As(OH)3 involves a slow hydrolysis of arsenic trioxide in water. ==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. :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==