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 fungic ...

with the formula CH₃AsO₃H₂. It is a colorless, water-soluble solid. Salts of this compound, e.g.

disodium methyl arsonate Disodium methyl arsonate (DSMA) is the organoarsenic compound with the formula CH3AsO3Na2. It is a colorless, water-soluble solid derived from methanearsonic acid. It is used as a herbicide. Tradenames include Metharsinat, Arrhenal, Disomear, ...

, 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 CH₃AsO₃H⁻ and .

Synthesis and biosynthesis

Reaction of

arsenous acid Arsenous acid (or arsenious 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 As(OH)3. ...

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 h ...

gives methylarsonic acid. This historically significant conversion is called the Meyer reaction: :As(OH)₃ + CH₃I + NaOH → CH₃AsO(OH)₂ + NaI + H₂O 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