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 arsine and arsenic acid. Despite their toxicity,
organoarsenic biomolecules are well known.
== History == Surprising for an area now considered of minor
importance, organoarsenic chemistry played a prominent role in the history of the field
of chemistry. 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 antibotics
(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) chemistry and uses === Arsenic(V) compounds typically feature the
functional groups RAsO(OH)2 or R2AsO(OH) (R = alkyl or aryl). Biomethylation of arsenic
compounds starts with the formation of methanearsonates. Thus, trivalent inorganic arsenic compounds
are methylated to give methanearsonate. S-adenosylmethionine is the methyl donor. The methanearsonates
are the precursors to dimethylarsonates, again by the cycle of reduction (to methylarsonous
acid) followed by a second methylation. This dimethyl compound is Cacodylic acid ((CH3)2AsO2H),
figures prominently throughout the chemistry of organoarsenic compounds. In contrast, the
dimethylphosphonic acid is less significant in the corresponding chemistry of phosphorus.
Cacodylic acid arises from the methylation of arsenic(III) oxide. 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) chemistry and uses ===
Most such 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. An important route to dimethylarsenic compounds
begin with reduction of cacodylic acid (see above):
(CH3)2AsO2H + 2 Zn + 4 HCl → (CH3)2AsH + 2 ZnCl2 + 2 H2O
(CH3)2AsO2H + SO2 + HI → (CH3)2AsI + SO3 + H2OA 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.
=== Organoarsenic(I) compounds and uses === Least significant in terms of commercial uses
and numbers are the organoarsenic(I) compounds. The anti-syphylic drugs Salvarsan and Neosalvarsan
are representative of this class. These compounds typically feature three bonds to As, but only
As-As single bonds. Compounds of arsenic(I) but containing As=As double bonds are rare.
== 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 == As arsenic is toxic to most life forms and
it occurs in elevated concentration in some areas several detoxification strategies have
evolved. Inorganic arsenic and its compounds, upon entering the food chain, are progressively
metabolized to a less toxic form of arsenic through a process of methylation. Organoarsenic
compounds arise via biomethylation of inorganic arsenic compounds, via processes mediated
by enzymes related to vitamin B12. For example, the mold Scopulariopsis brevicaulis produce
significant amounts of trimethylarsine if inorganic arsenic is present. 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.
Arsenobetaine was first identified in the Western rock lobster Carbohydrates bound to
arsenic, collectively known as arsenosugars, are found especially in seaweeds. Arsenic
containing lipids are also known. 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.Organoarsenic compounds may pose significant health hazards, depending on their
speciation, LD50s ranging from 5–6 mg/kg (very toxic) to 12000–15000 mg/kg (practically
non-toxic).
== Representative compounds == Some illustrative organoarsenic compound are
listed in the table below:
== See also == Arsenic biochemistry
Arsenic poisoning Arsenic toxicity
Category:Arsenic compounds