Selenols areorganic compounds that contain thefunctional group with the connectivityC−Se−H. Selenols are sometimes also calledselenomercaptans andselenothiols. Selenols are one of the principal classes oforganoselenium compounds.[1] A well-known selenol is theamino acidselenocysteine.
Selenols are structurally similar tothiols, but theC−Se bond is about 8% longer at 196pm. TheC−Se−H angle approaches 90°. The bonding involves almost purep-orbitals on Se, hence the near 90 angles. TheSe−Hbond energy is weaker than theS−H bond, consequently selenols are easilyoxidized and serve asH-atom donors. The Se-H bond is weaker than theS−H bond as reflected in their respectivebond dissociation energy (BDE). ForC6H5Se−H, the BDE is 326kJ/mol, while forC6H5S−H, the BDE is 368 kJ/mol.[2]
Selenols are about 1000 times strongeracids than thiols: thepKa ofCH3SeH is 5.2 vs 8.3 forCH3SH.Deprotonation affords theselenolateanion,RSe−, most examples of which are highlynucleophilic 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 strongervan 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 theSe−H bond. Theirconjugate bases, the selenolates, also have limited applications inorganic synthesis.
Selenols are important in certain biological processes. Three enzymes found in mammals contain selenols at their active sites:glutathione peroxidase,iodothyronine deiodinase, andthioredoxin reductase. The selenols in these proteins are part of theessential amino acidselenocysteine.[3] The selenols function as reducing agents to giveselenenic 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 incubatingselenomethionine with a bacterialmethionine gamma-lyase (METase)enzyme, by biologicalmethylation of selenide ion orin vivo by reduction ofmethaneseleninic 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 thanethaneselenol (CH3CH2SeH) or2-propaneselenol ((CH3)2CH(SeH)).[7]
Selenols are usually prepared by the reaction oforganolithium reagents orGrignard reagents with elemental Se.[8] For example,benzeneselenol is generated by the reaction ofphenylmagnesium bromide with selenium followed byacidification:[9]
Another preparative route to selenols involves thealkylation ofselenourea, followed byhydrolysis.[citation needed]
Selenols are often generated by reduction ofdiselenides followed byprotonation of the resulting selenolate:[citation needed]
Dimethyl diselenide can be easily reduced to methaneselenol within cells.[10]
Selenols are easily oxidized to diselenides, compounds containing anSe−Se bond. For example, treatment ofbenzeneselenol withbromine givesdiphenyl diselenide.
In the presence of base, selenols are readily alkylated to give selenides. This relationship is illustrated by themethylation of methaneselenol to givedimethylselenide.
Organoselenium compounds (or any selenium compound) are cumulative poisons despite the fact that trace amounts of Se are required for health.[11]