Benzeneselenol
Benzeneselenol, also known as selenophenol, is the organoselenium compound with the chemical formula Template:Chem2, often abbreviated PhSeH. It is the selenium analog of phenol. This colourless, malodorous compound is a reagent in organic synthesis.[1]
Synthesis
Benzeneselenol is prepared by the reaction of phenylmagnesium bromide and selenium:[2]
- PhMgBr + Se → PhSeMgBr
- PhSeMgBr + HCl → PhSeH + MgBrCl
Since benzeneselenol does not have a long shelf life, it is often generated in situ. A common method is by reduction of diphenyldiselenide. A further reason for this conversion is that often, it is the anion that is sought.[1]
Reactions
More so than thiophenol, benzeneselenol is easily oxidized by air. The facility of this reaction reflects the weakness of the Se-H bond, bond dissociation energy of which is estimated to be between 67 and 74 kcal/mol.[1] In contrast, the S-H BDE for thiophenol is near 80 kcal/mol.[3] The product is diphenyl diselenide as shown in this idealized equation:
The presence of the diselenide in benzeneselenol is indicated by a yellow coloration. The diselenide can be converted back to the selenol by reduction followed by acidification of the resulting Template:Chem2.
PhSeH is acidic with a pKa of 5.9. Thus at neutral pH, it is mostly ionized:
It is approximately seven times more acidic than the related thiophenol. Both compounds dissolve in water upon the addition of base. The conjugate base is Template:Chem2, a potent nucleophile.[1]
History
Benzeneselenol was first reported in 1888 by the reaction of benzene with selenium tetrachloride (Template:Chem2) in the presence of aluminium trichloride (Template:Chem2).[4][5]
Safety
The compound is intensely malodorous[6] and, like other organoselenium compounds, toxic.
References
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