Organoarsenic chemistry is the chemistry of compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called organoarsenicals, are produced industrially with uses as insecticides, herbicides, and fungicides. In general these applications are declining in step with growing concerns about their impact on the environment and human health. The parent compounds are arsane and arsenic acid. Despite the toxicity of arsenic metal, organoarsenic compounds can be found in nature.
History
Surprising for an area now considered of minor importance, organoarsenic chemistry played a prominent role in chemistry's history. The oldest known organoarsenic compound, the foul smelling cacodyl was reported in "cacodyl" (1760) and is sometimes classified as the first synthetic organometallic compound. The compound Salvarsan was one of the first pharmaceuticals, earning a Nobel prize for Paul Ehrlich. Various other organoarsenic compounds formerly found use as antibiotics (Solarson) or other medical uses.
Synthesis and classification Arsenic typically occurs in the oxidation states (III) and (V), illustrated by the halides AsX3 (X = F, Cl, Br, I) and AsF5. Correspondingly, organoarsenic compounds are commonly found in these two oxidation states. The hydroxyarsenic compounds are known:
arsonous acids (RAs(OH)2), rare (arsenous acid (As(OH)3) is well known) arsinous acids (R2AsOH), rare arsinic acids (R2As(O)OH), common, illustrated by cacodylic acid (R = CH3) arsonic acids (RAs(O)(OH)2), common, illustrated by phenylarsonic acid (R = C6H5)
Organoarsenic(V) compounds and uses Arsenic(V) compounds typically feature the functional groups RAsO(OH)2 or R2AsO(OH) (R = alkyl or aryl). Cacodylic acid, central to arsenic chemistry, arises from the methylation of arsenic(III) oxide. (In contrast, the dimethylphosphonic acid is less significant in the corresponding chemistry of phosphorus.) Phenylarsonic acids can be accessed by the reaction of arsenic acid with anilines, the so-called Bechamp reaction. The monomethylated acid, methanearsonic acid (CH3AsO(OH)2), is a precursor to fungicides (tradename Neoasozin) in the cultivation of rice and cotton. Derivatives of phenylarsonic acid (C6H5AsO(OH)2) are used as feed additives for livestock, including 4-hydroxy-3-nitrobenzenearsonic acid (3-NHPAA or Roxarsone), ureidophenylarsonic acid and p-arsanilic acid. These applications are controversial as they introduce soluble forms of arsenic into the environment. Compounds of arsenic(V) containing only organic ligands are rare, the pre-eminent member being the pentaphenyl derivative As(C6H5)5.
Organoarsenic(III) compounds and uses Many organoarsenic compounds are prepared by alkylation of AsCl3 and its derivatives using organolithium and Grignard reagents. For example, the series trimethylarsine ((CH3)3As), dimethylarsenic chloride ((CH3)2AsCl), and methylarsenic dichloride (CH3AsCl2) is known. Reduction of the chloride derivatives with hydride reducing reagents affords the corresponding hydrides, such as dimethylarsine ((CH3)2AsH) and methylarsine (CH3AsH2). Similar manipulations apply to other organoarsenic chloride compounds. Akin to the Direct process in organosilicon chemistry, methyl halides react with elemental As, as illustrated in the following idealized equation:
3 CH3Br + 2 As → (CH3)2AsBr + CH3AsBr2 Such reactions require copper catalysts, are conducted near 360 °C. Another route to dimethylarsenic compounds begins with reduction of the AsV compound cacodylic acid:
(CH3)2AsO2H + 2 Zn + 4 HCl → (CH3)2AsH + 2 ZnCl2 + 2 H2O (CH3)2AsO2H + SO2 + HI → (CH3)2AsI + SO3 + H2O A variety of heterocycles containing arsenic(III) are known. These include arsole, the arsenic analogue of pyrrole, and arsabenzene, the arsenic analogue of pyridine. Symmetrical organoarsenic(III) compounds, e.g. trimethylarsine and triphenylarsine, are commonly used as ligands in coordination chemistry. They behave like phosphine ligands, but are less basic. The diarsine C6H4(As(CH3)2)2, known as diars, is a chelating ligand. Thorin is an indicator for several metals.
Lower-order organoarsenic compounds and uses Per the double bond rule, compounds with As=As, As=C, and As≡C bonds are rare. They are observed in the gas phase but considerable steric protection is required to inhibit their conversion to oligomers as liquids or solids. Oligomers with As-As bonds include the anti-syphylic drugs Salvarsan and Neosalvarsan. They are typically tricoordinate at As and have formal oxidation state AsI. Small substituents, such as in (MeAs)n, equilibrate between three-, four-, and five-membered rings, but bulkier substituents usually settle on a four-membered configuration. Synthesis is typically by reductive dehalogenation with a metal.
Reactions Protic arsines oxidize to oligomers. For example, methylarsine oxidizes first to cyclo-methylarsine(I):
MeAsH2 + O → H2O + (MeAs)n These compounds have structures similar to the phosphorus sulfides. Arsenic-arsenic bonds are very weak, and oligomeric arsenic compounds are even more liable to oxidize than their hydrogenated precursors. The following reaction can, however, be prepared through electrochemical reduction in a zinc-sulfate cell. Oxidation first forms polymeric arsinoxides, e.g.:
MeAs + O → MeAsO Further oxidation then depolymerizes them to arsinous acids. Arsine(III) compounds add to multiple bonds as nucleophiles, but arsine(I) rings may instead insert the bond into the ring. In general, arsines are less Brønsted basic than phosphines (but more than stibines). Arsine ylides are generally less stable than phosphine ylides, decomposing spontaneously in the absence of a vicinal carbonyl. Stabilized ylides olefinate to a mixture of stereoisomers, whereas unstabilized ylides tend to epoxidate (like a Corey-Chaykovsky reagent). With enones, they either olefinate or cyclopropanate. With nitroso compounds, they either form imines or nitrones.
Chemical warfare Organoarsenic compounds, especially those featuring As-Cl bonds, have been used as chemical weapons, especially during World War I. Infamous examples include "Lewisite" (chlorovinyl-2-arsenic dichloride) and "Clark I" (chlorodiphenylarsine). "Phenyl Dick" (Phenyldichloroarsine) is another one.
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