Organoarsenic compound
Encyclopedia
Organoarsenic chemistry is the chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 of compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 containing a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

 between arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 and carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

s, herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...

s, and fungicide
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...

s. In general these applications are declining in step with growing concerns about their impact on the environment and human health. The parent compound is arsine
Arsine
Arsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...

. Despite their toxicity, organoarsenic biomolecule
Biomolecule
A biomolecule is any molecule that is produced by a living organism, including large polymeric molecules such as proteins, polysaccharides, lipids, and nucleic acids as well as small molecules such as primary metabolites, secondary metabolites, and natural products...

s 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
Cacodyl
Cacodyl, dicacodyl, tetramethyldiarsine, alkarsine or minor part of the "Cadet's fuming liquid" 2As—As2 is a poisonous oily liquid with a garlicky odor...

 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
Paul Ehrlich
Paul Ehrlich was a German scientist in the fields of hematology, immunology, and chemotherapy, and Nobel laureate. He is noted for curing syphilis and for his research in autoimmunity, calling it "horror autotoxicus"...

.

Synthesis and classification

Arsenic typically occurs in the oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

s (III) and (V), illustrated by the halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

s AsX3 (X = F, Cl, Br, I) and AsF5. Correspondingly, organoarsenic compounds are commonly found in these two oxidation states.

Organoarsenic(V) chemistry and uses

Arsenic(V) compounds typically feature the functional groups RAsO(OH)2 or R2AsO(OH) (R = alkyl or aryl). Cacodylic acid
Cacodylic acid
Cacodylic acid is the chemical compound with the formula 2AsO2H. Derivatives of cacodylic acid, cacodylates, were frequently used as herbicides. For example, "Agent Blue," one of the chemicals used during the Vietnam War, is a mixture of cacodylic acid and sodium cacodylate...

, with the formula (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
Arsenic acid
Arsenic acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but only found in solution where it...

 with aniline
Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...

s, the so-called Bechamp reaction
Bechamp reaction
In organic synthesis, the Bechamp reaction is used for producing arsonic acids from activated aromatic rings such as aniline. It was first described by A. J. Bechamp in 1863. The reaction is an electrophilic aromatic substitution, using arsenic acid as a reactant.One example of an important arsonic...

.

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
Phenylarsonic acid
Phenylarsonic acid is the chemical compound with the formula C6H5AsO2, commonly abbreviated PhAsO3H2. This colourless solid is an organic derivative of arsenic acid, AsO3, where one OH group has been replaced by a phenyl group. The compound is a buffering agent and a precursor to other...

 (C6H5AsO(OH)2) are used as feed additives for livestock, including 4-hydroxy-3-nitrobenzenearsonic acid
4-Hydroxy-3-nitrobenzenearsonic acid
4-Hydroxy-3-nitrobenzenearsonic acid is an organic compound that is widely used agriculturally as a chicken-feed additive. The molecule is a derivative of phenylarsonic acid . This organoarsenic compound, usually under the tradename Roxarsone, has attracted attention as a source of arsenic...

 (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
Trimethylarsine
Trimethylarsine is the chemical compound with the formula 3As, commonly abbreviated AsMe3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building block to other organoarsenic compounds, and serves as a ligand in coordination...

 ((CH3)3As), dimethylarsenic chloride ((CH3)2AsCl), and methylarsenic dichloride
Methyldichloroarsine
Methyldichloroarsine, sometimes abbreviated "MD", is an organoarsenic compound with the formula CH3AsCl2. This colourless volatile liquid is a highly toxic vesicant that has been used in chemical warfare.-History:...

  (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 route to dimethylarsenic compounds begin with reduction of cacodylic acid (see above):2AsO2H + 2 Zn + 4 HCl → (CH3)2AsH + 2 ZnCl2 + 2 H2O2AsO2H + SO2 + HI → (CH3)2AsI + SO3 + H2O

A variety of heterocycles containing arsenic(III) are known. These include arsole
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 atom...

, the arsenic analogue of pyrrole
Pyrrole
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...

, and arsabenzene
Arsabenzene
Arsabenzene is an organoarsenic heterocyclic compound with the chemical formula C5H5As. It belongs to a group of compounds called heteroarenes that have the general formula C5H5E ....

, the arsenic analogue of pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

.

Symmetrical organoarsenic(III) compounds, e.g. trimethylarsine
Trimethylarsine
Trimethylarsine is the chemical compound with the formula 3As, commonly abbreviated AsMe3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building block to other organoarsenic compounds, and serves as a ligand in coordination...

 and triphenylarsine
Triphenylarsine
Triphenylarsine is the chemical compound with the formula As3. This organoarsenic compound, often abbreviated AsPh3, is a colorless crystalline solid that is used as a ligand and a reagent in coordination chemistry and organic synthesis...

, 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
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

. Thorin
Thorin (chemistry)
Thorin is an indicator used in the determination of barium, beryllium, lithium, uranium and thorium compounds. Being a compound of arsenic, it is highly toxic....

 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
Neosalvarsan
Neosalvarsan is a synthetic chemotherapeutic that is an organoarsenic compound. It became available in 1912 and superseded the more toxic and less water-soluble salvarsan as an effective treatment for syphilis...

 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
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. Infamous examples include "Lewisite
Lewisite
Lewisite is an organoarsenic compound, specifically an arsine. It was once manufactured in the U.S. and Japan as a chemical weapon, acting as a vesicant and lung irritant...

" (chlorovinyl-2-arsenic dichloride) and "Clark I" (chlorodiphenylarsine). Phenyldichloroarsine
Phenyldichloroarsine
Phenyldichloroarsine, NATO abbreviation PD, is an organic arsenical vesicant and vomiting agent developed by Germany for use as a chemical warfare agent during World War I...

 is another one.

Organoarsenic compounds 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
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

, are progressively metabolized to a less toxic form of arsenic through a process of methylation
Methylation
In the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom...

. Organoarsenic compounds arise via biomethylation of inorganic arsenic compounds, via processes mediated by enzymes related to vitamin B12
Vitamin B12
Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins...

. For example the mold Scopulariopsis brevicaulis produce significant amounts of trimethylarsine
Trimethylarsine
Trimethylarsine is the chemical compound with the formula 3As, commonly abbreviated AsMe3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building block to other organoarsenic compounds, and serves as a ligand in coordination...

 if inorganic arsenic is present. The organic compound arsenobetaine
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...

, 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
Western rock lobster
Panulirus cygnus is a species of spiny lobster , found off the west coast of Australia. Panulirus cygnus is the basis of Australia's most valuable fishery, making up 20% of value of Australia's total fishing industry, and is identified as the western rock lobster.-Description:The species has five...

 

Carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s bound to arsenic, collectively known as arsenosugars, are found especially in seaweeds. Arsenic containing lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

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
Arsenicin A
Arsenicin A is a naturally occurring organoarsenic compound. It was first isolated from the New Caledonian marine sponge Echinochalina bargibanti...

.

Organoarsenic compounds may pose significant health hazards, depending extremely on their speciation (LD50 ranging from 5–6 mg /kg bw (very toxic) to 12000–15000 mg / kg bw (practically non-toxic).

Representative compounds

Some illustrative organoarsenic compound are listed in the table below:
Organoarsenic |R |Molar mass
Molar mass
Molar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...

CAS number Properties
10,10'-oxybis-10H-Phenoxarsine
502.2318 58-36-6
Triphenylarsine
Triphenylarsine
Triphenylarsine is the chemical compound with the formula As3. This organoarsenic compound, often abbreviated AsPh3, is a colorless crystalline solid that is used as a ligand and a reagent in coordination chemistry and organic synthesis...

Phenyl
306.23 603-32-7 Melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

 58-61 °C
Phenyldichloroarsine
Phenyldichloroarsine
Phenyldichloroarsine, NATO abbreviation PD, is an organic arsenical vesicant and vomiting agent developed by Germany for use as a chemical warfare agent during World War I...

phenyl, chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

222.93 696-28-6
Roxarsone 263.04 121-19-7
Arsenobetaine
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...

64436-13-1
Representative organoarsenic compounds

See also

  • Arsenic biochemistry
    Arsenic biochemistry
    Arsenic biochemistry refers to biochemical processes that can use arsenic or its compounds, such as arsenate. Arsenic is a moderately abundant element on the earth's crust, and although many arsenic compounds are often considered highly toxic, a wide variety of organoarsenic compounds are produced...

  • Arsenic poisoning
    Arsenic poisoning
    Arsenic poisoning is a medical condition caused by increased levels of the element arsenic in the body. Arsenic interferes with cellular longevity by allosteric inhibition of an essential metabolic enzyme...

  • Arsenic toxicity
    Arsenic toxicity
    Arsenic and many of its compounds are especially potent poisons. Arsenic disrupts ATP production through several mechanisms. At the level of the citric acid cycle, arsenic inhibits pyruvate dehydrogenase and by competing with phosphate it uncouples oxidative phosphorylation, thus inhibiting...


:Category:Arsenic compounds
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