Selenols are organic compounds that contain the functional group with the connectivity C−Se−H . Selenols are sometimes also called selenomercaptans and selenothiols. Selenols are one of the principal classes of organoselenium compounds. [1] A well-known selenol is the amino acid selenocysteine.
Selenols are structurally similar to thiols, but the C−Se bond is about 8% longer at 196 pm. The C−Se−H angle approaches 90°. The bonding involves almost pure p-orbitals on Se, hence the near 90 angles. The Se−H bond energy is weaker than the S−H bond, consequently selenols are easily oxidized and serve as H-atom donors. The Se-H bond is weaker than the S−H bond as reflected in their respective bond dissociation energy (BDE). For C6H5Se−H, the BDE is 326 kJ/mol, while for C6H5S−H, the BDE is 368 kJ/mol. [2]
Selenols are about 1000 times stronger acids than thiols: the pKa of CH3SeH is 5.2 vs 8.3 for CH3SH. Deprotonation affords the selenolate anion, RSe−, most examples of which are highly nucleophilic and rapidly oxidized by air. [3]
The boiling points of selenols tend to be slightly greater than for thiols. This difference can be attributed to the increased importance of stronger van der Waals bonding for larger atoms. Volatile selenols have highly offensive odors.
Selenols have few commercial applications, being limited by the toxicity of selenium as well as the sensitivity of the Se−H bond. Their conjugate bases, the selenolates, also have limited applications in organic synthesis.
Selenols are important in certain biological processes. Three enzymes found in mammals contain selenols at their active sites: glutathione peroxidase, iodothyronine deiodinase, and thioredoxin reductase. The selenols in these proteins are part of the essential amino acid selenocysteine. [3] The selenols function as reducing agents to give selenenic acid derivative (RSe−OH), which in turn are re-reduced by thiol-containing enzymes. Methaneselenol (commonly named "methylselenol") (CH3SeH), which can be produced in vitro by incubating selenomethionine with a bacterial methionine gamma-lyase (METase) enzyme, by biological methylation of selenide ion or in vivo by reduction of methaneseleninic acid (CH3−Se(=O)−OH), has been invoked to explain the anticancer activity of certain organoselenium compounds. [4] [5] [6] Precursors of methaneselenol are under active investigation in cancer prevention and therapy. In these studies, methaneselenol is found to be more biologically active than ethaneselenol (CH3CH2SeH) or 2-propaneselenol ((CH3)2CH(SeH)). [7]
Selenols are usually prepared by the reaction of organolithium reagents or Grignard reagents with elemental Se. For example, benzeneselenol is generated by the reaction of phenylmagnesium bromide with selenium followed by acidification: [8]
Another preparative route to selenols involves the alkylation of selenourea, followed by hydrolysis. Selenols are often generated by reduction of diselenides followed by protonation of the resulting selenolate:
Dimethyl diselenide can be easily reduced to methaneselenol within cells. [9]
Selenols are easily oxidized to diselenides, compounds containing an Se−Se bond. For example, treatment of benzeneselenol with bromine gives diphenyl diselenide.
In the presence of base, selenols are readily alkylated to give selenides. This relationship is illustrated by the methylation of methaneselenol to give dimethylselenide.
Organoselenium compounds (or any selenium compound) are cumulative poisons despite the fact that trace amounts of Se are required for health. [10]
Selenol, alcohol, thiol acidity order
Selenocysteine is the 21st proteinogenic amino acid. Selenoproteins contain selenocysteine residues. Selenocysteine is an analogue of the more common cysteine with selenium in place of the sulfur.
Glutathione peroxidase (GPx) is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage. The biochemical function of glutathione peroxidase is to reduce lipid hydroperoxides to their corresponding alcohols and to reduce free hydrogen peroxide to water.
A selenide is a chemical compound containing a selenium with oxidation number of −2. Similar to sulfide, selenides occur both as inorganic compounds and as organic derivatives, which are called organoselenium compound.
Organoselenium chemistry is the science exploring the properties and reactivity of organoselenium compounds, chemical compounds containing carbon-to-selenium chemical bonds. Selenium belongs with oxygen and sulfur to the group 16 elements or chalcogens, and similarities in chemistry are to be expected. Organoselenium compounds are found at trace levels in ambient waters, soils and sediments.
The Grieco elimination is an organic reaction describing the elimination reaction of an aliphatic primary alcohol through a selenide to a terminal alkene. It is named for Paul Grieco.
Selenium compounds are compounds containing the element selenium (Se). Among these compounds, selenium has various oxidation states, the most common ones being −2, +4, and +6. Selenium compounds exist in nature in the form of various minerals, such as clausthalite, guanajuatite, tiemannite, crookesite etc., and can also coexist with sulfide minerals such as pyrite and chalcopyrite. For many mammals, selenium compounds are essential. For example, selenomethionine and selenocysteine are selenium-containing amino acids present in the human body. Selenomethionine participates in the synthesis of selenoproteins. The reduction potential and pKa (5.47) of selenocysteine are lower than those of cysteine, making some proteins have antioxidant activity. Selenium compounds have important applications in semiconductors, glass and ceramic industries, medicine, metallurgy and other fields.
Benzeneselenol, also known as selenophenol, is the organoselenium compound with the chemical formula C6H5SeH, often abbreviated PhSeH. It is the selenium analog of phenol. This colourless, malodorous compound is a reagent in organic synthesis.
Diphenyl diselenide is the chemical compound with the formula (C6H5)2Se2, abbreviated Ph2Se2. This yellow-coloured solid is the oxidized derivative of benzeneselenol. It is used as a source of the PhSe unit in organic synthesis.
A selenonic acid is an organoselenium compound containing the −SeO3H functional group. The formula of selenonic acids is R−Se(=O)2−OH, where R is organyl group. Selenonic acids are the selenium analogs of sulfonic acids. Examples of the acid are rare. Benzeneselenonic acid PhSeO3H is a white solid. It can be prepared by the oxidation of benzeneselenol.
The enzyme selenocysteine lyase (SCL) (EC 4.4.1.16) catalyzes the chemical reaction
Selenourea is the organoselenium compound with the chemical formula Se=C(NH2)2. It is a white solid. This compound features a rare example of a stable, unhindered carbon-selenium double bond. The compound is used in the synthesis of selenium heterocycles. Selenourea is a selenium analog of urea O=C(NH2)2. Few studies have been done on the compound due to the instability and toxicity of selenium compounds. Selenourea is toxic if inhaled or consumed.
Ferredoxin-thioredoxin reductase EC 1.8.7.2, systematic name ferredoxin:thioredoxin disulfide oxidoreductase, is a [4Fe-4S] protein that plays an important role in the ferredoxin/thioredoxin regulatory chain. It catalyzes the following reaction:
Selenium is an essential micronutrient for animals, though it is toxic in large doses. In plants, it sometimes occurs in toxic amounts as forage, e.g. locoweed. Selenium is a component of the amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that functions as cofactor for glutathione peroxidases and certain forms of thioredoxin reductase. Selenium-containing proteins are produced from inorganic selenium via the intermediacy of selenophosphate (PSeO33−).
A selenenic acid is an organoselenium compound and an oxoacid with the general formula RSeOH, where R ≠ H. It is the first member of the family of organoselenium oxoacids, which also include seleninic acids and selenonic acids, which are RSeO2H and RSeO3H, respectively. Selenenic acids derived from selenoenzymes are thought to be responsible for the antioxidant activity of these enzymes. This functional group is sometimes called SeO-selenoperoxol.
A seleninic acid is an organoselenium compound and an oxoacid with the general formula RSeO2H, where R ≠ H. Its structure is R−Se(=O)−OH. It is a member of the family of organoselenium oxoacids, which also includes selenenic acids and selenonic acids, which are R−Se−OH and R−Se(=O)2−OH, respectively. The parent member of this family of compounds is methaneseleninic acid, also known as methylseleninic acid or "MSA".
Methaneseleninic acid is an organoselenium compound, a seleninic acid with the chemical formula CH3SeO2H. Its structure is CH3−Se(=O)−OH.
Bis(2-chloroethyl)selenide is the organoselenium compound with the formula Se(CH2CH2Cl)2. As a haloalkyl derivative of selenium, it is an analogue of bis(2-chloroethyl)sulfide, the prototypical sulfur mustard used in chemical warfare. Bis(2-chloroethyl)selenide has not been used as a chemical warfare agent, however it is still a potent alkylating agent and has potential in chemotherapy.
Organic selenocyanates are organoselenium compounds with the general formula RSeCN. They are generally colorless, air-stable solids or liquids with repulsive odors. In terms of structure, synthesis, and reactivity, selenocyanates and thiocyanates behave similarly.
Methaneselenol is the organoselenium compound with the formula CH3SeH. It is the simplest selenol. A colorless gas, it is notorious for its foul odor.
In chemistry, a selenosulfide refers to distinct classes of inorganic and organic compounds containing sulfur and selenium. The organic derivatives contain Se-S bonds, whereas the inorganic derivatives are more variable.