Methylarsonic acid is an

organoarsenic compound Organoarsenic chemistry is the chemistry of Chemical compound, compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticides, herb ...

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

disodium methyl arsonate Disodium methyl arsenate (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, M ...

, 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 arsorous 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 Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

+3 to +5. The

biomethylation Methylation, in the chemical sciences, is 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 terms ...

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 Methyl compounds