Article · Wikipedia archive · Last revised Jul 23, 2026

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.

Last revised
Jul 23, 2026
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≈ 2 min
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Source
Wikipedia ↗
Methylarsonic acid
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Names
Preferred IUPAC name
Methylarsonic acid1
Other names
Methanearsonic acid
Monomethylarsonic acid
Identifiers
3D model (JSmol)
4-04-00-03682
ChemSpider
ECHA InfoCard 100.004.278
EC Number
  • 204-705-6
RTECS number
  • PA1575000
UNII
UN number 1557
  • InChI=1S/CH5AsO3/c1-2(3,4)5/h1H3,(H2,3,4,5)
    Key: QYPPRTNMGCREIM-UHFFFAOYSA-N
  • C[As](=O)(O)O
Properties
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).

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

Reactions

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

Synthesis and biosynthesis

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

References

References

  1. International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. pp. 916, 918. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
  2. Grund, S. C.; Hanusch, K.; Wolf, H. U. "Arsenic and Arsenic Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_113.pub2. ISBN 978-3-527-30673-2.
  3. G. Meyer (1883). "Ueber einige anomale Reaktionen". Berichte der deutschen chemischen Gesellschaft. 13: 1439–1443. doi:10.1002/cber.188301601316.
  4. Styblo, M.; Del Razo, L. M.; Vega, L.; Germolec, D. R.; LeCluyse, E. L.; Hamilton, G. A.; Reed, W.; Wang, C.; Cullen, W. R.; Thomas, D. J. (2000). "Comparative toxicity of trivalent and pentavalent inorganic and methylated arsenicals in rat and human cells". Archives of Toxicology. 74: 289–299. doi:10.1007/s002040000134.