Methylarsonic acid is an
organoarsenic compound with the formula CH
3AsO
3H
2. It is a colorless, water-soluble solid. Salts of this compound, e.g.
disodium methyl arsonate, have been widely used in as herbicides and fungicides in growing cotton and rice.
Reactions
Near physiological pH, methanearsonic acid converts to its conjugate bases, the methylarsonates. These include CH
3AsO
3H
− and .
Synthesis and biosynthesis
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 ...
gives methylarsonic acid. This historically significant conversion is called the Meyer reaction:
:As(OH)
3 + CH
3I + NaOH → CH
3AsO(OH)
2 + NaI + H
2O
The then-novel aspect of the reaction was that alkylation occurs at arsenic, leading to oxidation of arsenic from
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. ...
+3 to +5.
The
biomethylation of arsenic compounds is thought to start with the formation of methanearsonates. Thus, trivalent arsenic compounds are methylated to give methanearsonate.
''S''-Adenosylmethionine is the methyl donor. The methanearsonates are the precursors to
cacodylates, again by the cycle of reduction (to methylarsonous acid) followed by a second methylation.
[
]
Safety
Like most arsenic compounds, it is highly toxic.
References
{{Reflist
Arsonic acids