Methylarsonic acid

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, have been widely used in as herbicides and fungicides in growing cotton and rice.^[2]

Methylarsonic acid

Names
Preferred IUPAC name Methylarsonic acid[1]
Other names Methanearsonic acid Monomethylarsonic acid
Identifiers
CAS Number	124-58-3
3D model (JSmol)	Interactive image
Beilstein Reference	4-04-00-03682
ChemSpider	8604
ECHA InfoCard	100.004.278
EC Number	204-705-6
PubChem CID	8948
RTECS number	PA1575000
UNII	J37VJ5709S
UN number	1557
CompTox Dashboard (EPA)	DTXSID4020903
InChI InChI=1S/CH5AsO3/c1-2(3,4)5/h1H3,(H2,3,4,5) Key: QYPPRTNMGCREIM-UHFFFAOYSA-N
SMILES C[As](=O)(O)O
Properties
Chemical formula	CH5AsO3
Molar mass	139.970 g·mol−1
Appearance	white solid
Melting point	160.5 °C (320.9 °F; 433.6 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Reactions

Near physiological pH, methanearsonic acid converts to its conjugate bases, the methylarsonates. These include CH₃AsO₃H⁻ and CH
₃AsO²⁻
₃.

Synthesis and biosynthesis

Not to be confused with Meyer synthesis.

Reaction of arsenous acid with methyl iodide gives methylarsonic acid. This historically significant conversion is called the Meyer reaction:^[3]

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 +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.^[4]

Safety

Like most arsenic compounds, it is highly toxic.^[4]