
Benzeneselenol

Benzeneselenol, also known as selenophenol, is the organoselenium compound with the formula C6H5SeH, often abbreviated PhSeH. It is the selenium analog of phenol. This colourless, intensely malodorous compound is a useful reagent in organic synthesis.
Synthesis.
Benzeneselenol is prepared by the reaction of phenylmagnesium bromide and selenium:
Reactions.
More so than thiophenol, benzeneselenol is easily oxidized by air. The facility of this reaction reflects the weakness of the Se-H bond. 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 PhSe−.
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 PhSe−, a potent nucleophile.
History.
Benzeneselenol was first reported in 1888 by the reaction of benzene with selenium tetrachloride (SeCl4) in the presence of aluminium trichloride (AlCl3).
Safety.
The compound is intensely malodorous and, like other organoselenium compounds, toxic.