Methylarsonic acid is an
organoarsenic compound
Organoarsenic chemistry is the chemistry of compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticides, herbicides, and fu ...
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 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. C ...
+3 to +5.
The
biomethylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These ...
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
cacodylate
Cacodylic acid is an organoarsenic compound with the formula (CH3)2 AsO2H. With the formula R2As(O)OH, it is the simplest of the arsinic acids. It is a colorless solid that is soluble in water.
Neutralization of cacodylic acid with base gives cac ...
s, 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