Organoselenium compound

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One of a group of compounds that contain both selenium (Se) and carbon (C) and frequently other elements as well, for example, halogen, oxygen (O), sulfur (S), or nitrogen (N). Organoselenium compounds have become common in organic chemistry laboratories, where they have numerous applications, particularly in the area of organic synthesis. Organoselenium compounds formally resemble their sulfur analogs and may be classified similarly. For instance, selenols, selenides, and selenoxides are clearly related to thiols, sulfides, and sulfoxides. Despite structural similarities, however, sulfur and selenium compounds are often strikingly different with respect to their stability, properties, and ease of formation. See also Organosulfur compound; Selenium.

Selenols have the general formula RSeH, where R represents either an aryl or alkyl group. They are prepared by the alkylation of hydrogen selenide (H₂Se) or selenide salts, as well as by the reaction of Grignard reagents or organolithium compounds with selenium, as in reactions (1).
1

Selenols are stronger acids than thiols. They and their conjugate bases are powerful nucleophiles that react with alkyl halides (R′X; R′ = alkyl group) or similar electrophiles to produce selenides (RSeR′); further alkylation yields selenonium salts, as shown in reactions (2),
2

where R″ represents an alkyl group that may be different from the alkyl group R′. Both acyclic and cyclic selenides are known. See also Electrophilic and nucleophilic reagents; Grignard reaction; Reactive intermediates.

Diselenides (RSeSeR) are usually produced by the aerial oxidation of selenols; they react in turn with chlorine or bromine, yielding selenenyl halides or selenium trihalides. Diselenides are easily oxidized to seleninic acids or anhydrides by reagents such as hydrogen peroxide or nitric acid.

Selenoxides are readily obtained from the oxidation of selenides with hydrogen peroxide (H₂O₂) or similar oxidants. Selenoxides undergo facile elimination to produce olefins and selenenic acids, a process known as syn-elimination.

Selenocarbonyl compounds tend to be considerably less stable than their thiocarbonyl or carbonyl counterparts because of the weaker double bond between the carbon and selenium atoms. Selenoamides, selenoesters and related compounds can be isolated, but selenoketones (selones) and selenoaldehydes are highly unstable. See also Aldehyde; Amide; Chemical bonding; Ester; Ketone.

The charge-transfer complexes formed between certain Organoselenium donor molecules and appropriate acceptors such as tetracyanoquinodimethane are capable of conducting electric current. Selenium-containing polymers are also of interest as organic conductors. See also Coordination complexes; Organic conductor.

Selenium is an essential trace element, and its complete absence in the diet is severely detrimental to human and animal health. The element is incorporated into selenoproteins such as glutathione peroxidase, which acts as a natural antioxidant. See also Bioinorganic chemistry; Coordination chemistry; Protein.

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