Arsenic is the third member of the nitrogen family of elements and hence possesses an outermost shell having the electron configuration of 4
s
2
4
p
3
. The most common oxidation states of arsenic are
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,
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, and
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, although compounds containing the simple
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,
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, and
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ions are unknown. In the majority of arsenic compounds the arsenic atom is in the tetrahedral valence state. Arsenic compounds have numerous practical applications. Although a variety of inorganic and organic arsenicals are used in commerce, arsenic trioxide, As
2
O
3
, accounted for 98% of the arsenic consumed in 1988. The commercial uses of arsenic compounds in 1988 were wood preservatives, agricultural products (herbicides and desiccants), glass, nonferrous alloys and electronics, and animal feed additives and pharmaceuticals. Chromated copper arsenate (CCA) is the most widely used arsenic‐based wood preservative. Arsenic compounds must be considered extremely poisonous. Arsine, AsH
3
, is a colorless, exceedingly poisonous gas with an unpleasant garlic‐like odor, soluble to the extent of 2 mL at 101 kPa (1 atm) per 100 g of water at RT. Arsine is formed when any inorganic arsenic‐bearing material is brought in contact with zinc and sulfuric acid. Arsine is a good reducing agent, capable of reducing many substances. Arsine is used for the preparation of gallium arsenide, GaAs, and there are numerous patents covering this subject. Arsenic forms a complete series of trihalides, but arsenic pentafluoride is the only well‐known simple pentahalide. The trihalides may be prepared by direct combination of the elements. The only arsenic oxides of commercial importance are the trioxide and the pentoxide. Arsenic trioxide (arsenic(III) oxide, arsenic sesquioxide, arsenous oxide, white arsenic, arsenic), As
2
O
3
, is the most important arsenic compound of commerce. Arsenic trioxide may be made by burning arsenic in air or by the hydrolysis of an arsenic trihalide. Commercially, it is obtained by roasting arsenopyrite, FeAsS. Commercial arsenic acid corresponds to the composition, one mole of arsenic pentoxide to four moles of water. Arsenic combines readily with carbon to form a wide variety of compounds containing one or more As–C bonds. These may be broadly divided into As(III) and As(V) compounds. The As(III) compounds contain from one to four organic groups; the As(V) compounds from one to six organic groups. Primary arsines are commonly prepared by the zinc–hydrochloric acid reduction of substances containing one organic group attached to arsenic (such as arsonic acids, dihaloarsines, and compounds with arsenic–arsenic bonds). Secondary arsines are obtained in good yields by the reduction of arsinic acids or haloarsines with amalgamated zinc and hydrochloric acid. These compounds are extremely sensitive to oxygen, and in some cases are spontaneously inflammable in air. They readily undergo addition reactions with alkenes, alkynes, aldehydes, ketones, isocyanates, and azo compounds. An enormous number of trialkyl‐ and triarylarsines are known. They are usually prepared by the reaction of an organometallic compound with an arsenic trihalide, a haloarsine, or a dihaloarsine. Most trialkylarsines are volatile liquids with intensely disagreeable odors. They react readily with oxygen, and in some cases they ignite spontaneously when exposed to air. Triarylarsines are solids that can usually be handled in air without danger of oxidation. They are, however, easily converted to triarylarsine oxides with suitable oxidizing agents. Tertiary arsines have been widely employed as ligands in a variety of transition metal complexes. Haloarsines, dihaloarsines, and related compounds, and diarsines and diarsenes have been obtained. A number of organoarsenic compounds containing rings of four, five, or six arsenic atoms have been prepared (cyclic polyarsines). A large number of polymeric substances, (RAs)
x
or (ArAs)
x
, are also known. They are usually prepared by the reduction of arsonic acids with hypophosphorous acid or sodium dithionite. Arsenin (arsabenzene), C
5
H
5
As, the arsenic analogue of pyridine, can be prepared by the treatment of 1,4‐dihydro‐1,1‐dibutylstannabenzene with arsenic trichloride. A large number of arsenin derivatives have also been studied. The arsonic acids, compounds of the type RAsO (OH)
2
, are among the most important organic arsenicals. Both arsonic and arsinic acids give precipitates with many metal ions, a property which has found considerable use in analytical chemistry. Of particular importance are certain azo dyes containing both arsonic and sulfonic acid groups. Both aliphatic and aromatic arsine oxides, compounds of the type R
3
AsO, are well known. The products are extremely hygroscopic and can only be handled under dry box conditions. Halides of the types RAsX
4
, R
2
AsX
3
, R
3
AsX
2
, and R
4
AsX are known. The R
4
AsX compounds are ionic in nature and are discussed under arsonium salts. Arsonium salts are compounds of the type R
4
AsY, where R may be either an alkyl or aryl group and Y is a wide variety of negative groups, such as halogen, nitrate, sulfate, and perchlorate. Arsonium salts have found considerable use in analytical chemistry. Compounds of the type R
5
As, where R may be aliphatic or aromatic, have assumed considerable importance in arsenic chemistry. A large number of pentaarylarsoranes have been prepared, including a number of spirocyclic compounds. These compounds are of particular interest in studies on the stereochemistry of five‐covalent compounds. Since 1943, when penicillin was shown to be effective for the therapy of syphilis, there has been much less work on the use of organoarsenic compounds in medicine. No important new arsenical drug has been introduced. However, arsenicals are still important for the treatment of African trypanosomiasis; they are probably indispensable for the late neurological stage of the disease. Arsenamide (thiacetarsamide), C
11
H
12
AsNO
5
, is a thioarsenite that is employed in veterinary medicine. Although relatively toxic, it has proved useful for the treatment of
Dirofilaria immitis
(heartworm) infestation in dogs.
