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Organoarsenic chemistry is the
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
of compounds containing a
chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
between
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, b ...
and
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as
insecticide Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed t ...
s,
herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page f ...
s, and
fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality ...
s. 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 Arsine (IUPAC name: arsane) is an inorganic compound with the formula As H3. This flammable, pyrophoric, and highly toxic pnictogen hydride gas is one of the simplest compounds of arsenic. Despite its lethality, it finds some applications in th ...
and
arsenic acid Arsenic acid or trihydrogen arsenate is the chemical compound with the formula . More descriptively written as , this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as su ...
. Despite their toxicity, organoarsenic
biomolecule A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or developmental biology, development. Biom ...
s are well known.


History

140px, Cacodyl (tetramethyldiarsine) was one of the first organoarsenic compounds. 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 Cacodyl, also known as dicacodyl or tetramethyldiarsine, (CH3)2As–As(CH3)2, is an organoarsenic compound that constitutes a major part of " Cadet's fuming liquid" (named after the French chemist Louis Claude Cadet de Gassicourt). It is a poisono ...
was reported in "cacodyl" (1760) and is sometimes classified as the first synthetic
organometallic compound Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and s ...
. The compound Salvarsan was one of the first pharmaceuticals, earning a Nobel prize for
Paul Ehrlich Paul Ehrlich (; 14 March 1854 – 20 August 1915) was a Nobel Prize-winning German physician and scientist who worked in the fields of hematology, immunology, and antimicrobial chemotherapy. Among his foremost achievements were finding a cure ...
. Various other organoarsenic compounds formerly found use as antibiotics (Solarson) or other medical uses.


Synthesis and classification

Arsenic typically occurs in the
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s (III) and (V), illustrated by the
halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a flu ...
s 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 Arsinic acids are organoarsenic compounds with the formula R2AsO2H. They are formally, but not actually, related to arsinic acid, a hypothetical compound of the formula H2AsO2H. Arsinic acids are monoprotic, weak acids. They react with sodium s ...
(R2As(O)OH), common, illustrated by
cacodylic acid Cacodylic acid is an organoarsenic compound with the formula (CH3)2 AsO2H. With the formula R2As(O)OH, it is the simplest of the arsinic acids. It is a colorless solid that is soluble in water. Neutralization of cacodylic acid with base gives cac ...
(R = CH3) *
arsonic acids Arsonic acids are a subset of organoarsenic compounds defined as oxyacids where a pentavalent arsenic atom is bonded to two hydroxyl groups, a third oxygen atom (this one with a double bond), and an organic substituent. The salts/conjugate bases of ...
(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). 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 Cacodylic acid is an organoarsenic compound with the formula (CH3)2 AsO2H. With the formula R2As(O)OH, it is the simplest of the arsinic acids. It is a colorless solid that is soluble in water. Neutralization of cacodylic acid with base gives cac ...
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 Arsenic acid or trihydrogen arsenate is the chemical compound with the formula . More descriptively written as , this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as su ...
with
aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starti ...
s, 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.Elschenbroich, C. "Organometallics" (2006) Wiley-VCH: Weinheim.


Organoarsenic(III) compounds and uses

Most such compounds are prepared by alkylation of AsCl3 and its derivatives using
organolithium In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...
and
Grignard reagent A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide . ...
s. For example, the series
trimethylarsine Trimethylarsine (abbreviated TMA or TMAs) is the chemical compound with the formula (CH3)3As, commonly abbreviated As Me3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building bloc ...
((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 + H2O A variety of heterocycles containing arsenic(III) are known. These include
arsole Arsole, also called arsenole or arsacyclopentadiene, is an organoarsenic compound with the formula C4H4AsH. It is classified as a metallole and is isoelectronic to and related to pyrrole except that an arsenic atom is substituted for the nitrogen ...
, the arsenic analogue of
pyrrole Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4 H4 NH. It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-meth ...
, and arsabenzene, the arsenic analogue of
pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...
. Symmetrical organoarsenic(III) compounds, e.g.
trimethylarsine Trimethylarsine (abbreviated TMA or TMAs) is the chemical compound with the formula (CH3)3As, commonly abbreviated As Me3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building bloc ...
and
triphenylarsine Triphenylarsine is the chemical compound with the formula As(C6H5)3. This organoarsenic compound, often abbreviated As Ph3, is a colorless crystalline solid that is used as a ligand and a reagent in coordination chemistry and organic synthesis. Th ...
, are commonly used as ligands in
coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Man ...
. They behave like phosphine ligands, but are less basic. The diarsine C6H4(As(CH3)2)2, known as diars, is a chelating
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
. 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.


Arsaalkenes and arsaalkynes

Following the pattern described by the
double bond rule In chemistry, the double bond rule states that elements with a principal quantum number greater than 2 for their valence electrons ( period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). The double ...
, compounds with As=As, As=C, and As≡C bonds are rare. They are observed in the gas phase but as liquids or solids, considerable steric protection is required to inhibit their conversion to oligomers.


Chemical warfare

Organoarsenic compounds, especially those featuring As-Cl bonds, have been used as
chemical weapon A chemical weapon (CW) is a specialized munition that uses chemicals formulated to inflict death or harm on humans. According to the Organisation for the Prohibition of Chemical Weapons (OPCW), this can be any chemical compound intended as a ...
s, especially during
World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...
. Infamous examples include "
Lewisite Lewisite (L) (A-243) is an organoarsenic compound. It was once manufactured in the U.S., Japan, Germany and the Soviet Union for use as a chemical weapon, acting as a vesicant (blister agent) and lung irritant. Although the substance is colorless ...
" (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 A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), de ...
, are progressively metabolized to a less toxic form of arsenic through a process of
methylation In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These ...
. Organoarsenic compounds arise via biomethylation of inorganic arsenic compounds, via processes mediated by enzymes related to vitamin B12. For example, the mold '' Scopulariopsis brevicaulis'' produces significant amounts of
trimethylarsine Trimethylarsine (abbreviated TMA or TMAs) is the chemical compound with the formula (CH3)3As, commonly abbreviated As Me3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building bloc ...
if inorganic arsenic is present. The organic compound
arsenobetaine Arsenobetaine is an organoarsenic compound that is the main source of arsenic found in fish. It is the arsenic analog of trimethylglycine, commonly known as betaine. The biochemistry and its biosynthesis are similar to those of choline and betaine ...
, 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
Saccharide In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s bound to arsenic, collectively known as arsenosugars, are found especially in seaweeds. Arsenic containing
lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
s 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 Caledonia ) , anthem = "" , image_map = New Caledonia on the globe (small islands magnified) (Polynesia centered).svg , map_alt = Location of New Caledonia , map_caption = Location of New Caledonia , mapsize = 290px , subdivision_type = Sovereign st ...
n marine sponge ''Echinochalina bargibanti''. Organoarsenic compounds may pose significant health hazards, depending on their speciation. Arsenous acid (As(OH)3) has an
LD50 In toxicology, the median lethal dose, LD50 (abbreviation for "lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a toxin, radiation, or pathogen. The value of LD50 for a substance is the ...
of 34.5 mg/kg (mice) whereas for the betaine (CH3)3As+CH2CO2 the LD50 exceeds 10 g/kg.


Representative compounds

Some illustrative organoarsenic compound are listed in the table below:


Nomenclature

The naming of
cyclic Cycle, cycles, or cyclic may refer to: Anthropology and social sciences * Cyclic history, a theory of history * Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr. * Social cycle, various cycles in so ...
organoarsenic compounds is based on an extension of the
Hantzsch–Widman nomenclature In organic chemistry, Hantzsch–Widman nomenclature, also called the extended Hantzsch–Widman system (named for Arthur Rudolf Hantzsch and ), is a type of systematic chemical nomenclature used for naming heterocyclic parent hydrides having no ...
system approved by
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
, as summarized below: Because of its similarity to the English slang word " arsehole" (in common use outside North America), the name "
arsole Arsole, also called arsenole or arsacyclopentadiene, is an organoarsenic compound with the formula C4H4AsH. It is classified as a metallole and is isoelectronic to and related to pyrrole except that an arsenic atom is substituted for the nitrogen ...
" has been considered a target of fun, a "silly name",Paul W May, ''Molecules with Silly or Unusual Names'', publ. 2008 Imperial College Press, (pbk). See also the Web page
Molecules with Silly or Unusual Names
at the School of Chemistry, University of Bristol, (retrieved 29 Sept 2008)
and one of several chemical compounds with an unusual name. However, this "silly name" coincidence has also stimulated detailed scientific studies.


See also

* Arsenic biochemistry *
Arsenic poisoning Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
*
Arsenic toxicity Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...
* :Arsenic compounds


References

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