Organoarsenic chemistry explained

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 their toxicity, organoarsenic biomolecules are well known.

History

140px|thumb|right|Cacodyl (tetramethyldiarsine) was one of the first organoarsenic compounds.[1]

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.[2]

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:

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.[3]

Organoarsenic(III) compounds and uses

Many organoarsenic compounds are prepared by alkylation of AsCl3 and its derivatives using organolithium and Grignard reagents.[3] 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:[4]

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), 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.[5] For example, methylarsine oxidizes first to cyclo-methylarsine(I):

MeAsH2 + O → H2O + (MeAs)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 → MeAsOFurther 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). Phenyldichloroarsine is another one.

In nature

The organic compound arsenobetaine, a betaine, is found in some marine foods such as fish and algae, and also in mushrooms in larger concentrations. The average person's intake is about 10-50 μg/day. Values about 1000 μg are not unusual following consumption of fish or mushrooms. But there is little danger in eating fish because this arsenic compound is nearly non-toxic.[11] Arsenobetaine was first identified in the Western rock lobster[12] [13]

Saccharides bound to arsenic, collectively known as arsenosugars, are found especially in seaweeds. Arsenic containing lipids are also known.[14] Although arsenic and its compounds are toxic for humans, one of the first synthetic antibiotics was Salvarsan, the use of which has long been discontinued.

The only polyarsenic compound isolated from a natural source is arsenicin A, found in the New Caledonian marine sponge Echinochalina bargibanti.[15]

Organoarsenic compounds may pose significant health hazards, depending on their speciation. Arsenous acid (As(OH)3) has an LD50 of 34.5 mg/kg (mice) whereas for the betaine (CH3)3As+CH2CO2 the LD50 exceeds 10 g/kg.[11]

Representative compounds

Some illustrative organoarsenic compound are listed in the table below:

Representative organoarsenic compounds [16] !Name!!Substituents!!Structure!!Molar mass!!CAS number!!Properties
10,10'-oxybis-10H-PhenoxarsineAryl502.2318 58-36-6
TriphenylarsinePhenyl306.23603-32-7 Melts 58-61 °C
PhenyldichloroarsinePhenyl, chlorine222.93696-28-6
RoxarsonePhenyl, oxygen263.04121-19-7
ArsenobetaineMethyl64436-13-1
Arsenicin AMethyl, oxygen389.76925705-41-5Melts 182C184C

See also

Notes and References

  1. Cadet's Fuming Arsenical Liquid and the Cacodyl Compounds of Bunsen. Dietmar . Seyferth. Organometallics. 2001. 20. 8. 1488 - 1498. 10.1021/om0101947. free.
  2. Singh, R. Synthetic Drugs. Mittal Publications (2002).
  3. Elschenbroich, C. "Organometallics" (2006) Wiley-VCH: Weinheim.
  4. 10.1016/0022-1902(61)80492-2 . The direct synthesis of organoarsenic and organoantimony compounds . 1961 . Maier . L. . Rochow . E.G. . Fernelius . W.C. . Journal of Inorganic and Nuclear Chemistry . 16 . 3–4 . 213–218 .
  5. Book: The Chemistry of Organic Arsenic, Antimony, and Bismuth Compounds . Wiley . 1994 . 047193044X . Patai . Saul . Saul Patai . Chemistry of Functional Groups . Chichester, UK . 10.1002/0470023473.
  6. Reimer. K. J.. Koch, I. . Cullen, W. R. . 2010. Organoarsenicals. Distribution and transformation in the environment. Metal Ions in Life Sciences. RSC publishing. Cambridge. 7 . 165–229. 978-1-84755-177-1. 10.1039/9781849730822-00165. 20877808.
  7. Dopp. E.. Kligerman, A. D. . Diaz-Bone, R. A. . 2010. Organoarsenicals. Uptake, metabolism and toxicity. Metal Ions in Life Sciences. RSC publishing. Cambridge. 7. 231–265. 978-1-84755-177-1. 20877809. 10.1515/9783110436600-012.
  8. Biomethylation of Arsenic in an Arsenic-rich Freshwater Environment . Toshikazu Kaise . Mitsuo Ogura . Takao Nozaki . Kazuhisa Saitoh . Teruaki Sakurai . Chiyo Matsubara . Chuichi Watanabe . Ken'ichi Hanaoka . Applied Organometallic Chemistry . 11 . 4. 297 - 304 . 1998 . 10.1002/(SICI)1099-0739(199704)11:4<297::AID-AOC584>3.0.CO;2-0. free .
  9. Ronald . Bentley . Chasteen, Thomas G. . Microbial Methylation of Metalloids: Arsenic, Antimony, and Bismuth . Microbiology and Molecular Biology Reviews . 2002 . 66 . 2 . 250 - 271 . 10.1128/MMBR.66.2.250-271.2002 . 12040126 . 120786.
  10. 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 . . 74 . 6 . 289–299 . 10.1007/s002040000134 . 11005674 . 1025140.
  11. Arsenic speciation in the environment . William R. . Cullen . Reimer, Kenneth J. . Chemical Reviews . 1989 . 89 . 4 . 713 - 764 . 10.1021/cr00094a002. 10214/2162 . free .
  12. Arsenic Species in Marine Samples . Kevin A. . Francesconi . John S. . Edmonds . Croatica Chemica Acta . 71 . 2 . 343 - 359 . 1998 . dead . https://web.archive.org/web/20080309143755/http://public.carnet.hr/ccacaa/CCA-PDF/cca1998/v71-n2/CCA_71_1998_343_359_FRANCES.pdf . 2008-03-09 .
  13. Isolation, crystal structure and synthesis of arsenobetaine, the arsenical constituent of the western rock lobster panulirus longipes cygnus George . John S. Edmonds . Kevin A. Francesconi . Jack R. Cannon. Colin L. Raston. Colin L. Raston . Brian W. Skelton . Allan H. White . amp . Tetrahedron Letters . 18 . 18 . 1543–1546 . 1977 . 10.1016/S0040-4039(01)93098-9 .
  14. Arsenic-Containing Long-Chain Fatty Acids in Cod-Liver Oil: A Result of Biosynthetic Infidelity? . Alice Rumpler . John S. Edmonds . Mariko Katsu . Kenneth B. Jensen . Walter Goessler . Georg Raber . Helga Gunnlaugsdottir . Kevin A. Francesconi . Angew. Chem. Int. Ed. . 47 . 14. 2665 - 2667 . 2008 . 10.1002/anie.200705405 . 18306198.
  15. 10.1002/chem.200600783 . On the First Polyarsenic Organic Compound from Nature: Arsenicin a from the New Caledonian Marine SpongeEchinochalina bargibanti . 2006 . Mancini . Ines . Guella . Graziano . Frostin . Maryvonne . Hnawia . Edouard . Laurent . Dominique . Debitus . Cecile . Pietra . Francesco . Chemistry: A European Journal . 12 . 8989–94 . 17039560 . 35.
  16. Web site: Home . sigmaaldrich.com.