Methylarsonic acid

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Methylarsonic acid
Preferred IUPAC name
Methylarsonic acid[1]
Other names
Methanearsonic acid
Monomethylarsonic acid
3D model (JSmol)
ECHA InfoCard 100.004.278 Edit this at Wikidata
EC Number
  • 204-705-6
RTECS number
  • PA1575000
UN number 1557
  • InChI=1S/CH5AsO3/c1-2(3,4)5/h1H3,(H2,3,4,5)
  • C[As](=O)(O)O
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]


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

Synthesis and biosynthesis[edit]

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]


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


  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.
  3. ^ G. Meyer (1883). "Ueber einige anomale Reaktionen". Berichte der deutschen chemischen Gesellschaft. 13: 1439–1443. doi:10.1002/cber.188301601316.
  4. ^ a b 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.