Methylarsonic acid

Methylarsonic acid
Names
	Other names methanearsonic 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

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 a herbicides and fungicides in growing cotton and rice.[1]

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:[2]

As(OH)₃ + CH₃I + NaOH → CH₃AsO(OH)₂ + NaI + H₂O

The then novel aspect of the reaction was that alkylation occurs at As, leading to oxidation of As from III to V.

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.[3]

Safety

Like all arsenic compounds, it is highly toxic.[3]

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References

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.CS1 maint: multiple names: authors list (link)
G. Meyer (1883). "Ueber einige anomale Reaktionen". Berichte der deutschen chemischen Gesellschaft. 13: 1439–1443. doi:10.1002/cber.188301601316.
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.

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[Ullmann-1] 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.CS1 maint: multiple names: authors list (link)

[2] G. Meyer (1883). "Ueber einige anomale Reaktionen". Berichte der deutschen chemischen Gesellschaft. 13: 1439–1443. doi:10.1002/cber.188301601316.

[Cullen-3] 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.