Methylarsonic acid is an organoarsenic compound with the formula CH3AsO3H2. It is a colorless, water-soluble solid. Salts of this compound, e.g. disodium methyl arsonate (DSMA), have been widely used as herbicides and fungicides for cotton and rice.
Reactions At physiological pH, methylarsonic acid exists mainly as the monoanion CH3AsO3H−; the dianion predominates above about pH 9.
Synthesis and biosynthesis
Reaction of sodium arsenite with methyl iodide gives methylarsonic acid. This historically significant conversion is called the Meyer reaction: NaAsO(OH)2 + CH3I → CH3AsO(OH)2 + NaI The then-novel aspect of the reaction was that alkylation occurs at arsenic, leading to oxidation of arsenic from oxidation state +3 to +5.
The biomethylation of inorganic trivalent arsenic is thought to begin with its conversion to methylarsonate, with S-adenosylmethionine as the methyl donor. Methylarsonates are the precursors to cacodylates by reduction (to methylarsonous acid) followed by a second methylation.
Safety Methylarsonic acid is much less toxic than inorganic arsenic, but its metabolite methylarsonous acid is highly toxic.
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