Benzeneselenol, also known as selenophenol, is the

organoselenium compound Organoselenium compounds (or seleno-organic) are chemical compounds containing carbon-to-selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity. Selenium belongs with oxygen and sulf ...

with the formula C₆H₅SeH, often abbreviated PhSeH. It is the selenium analog of

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it req ...

. This colourless, malodorous compound is a reagent in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

Synthesis

Benzeneselenol is prepared by the reaction of

phenylmagnesium bromide Phenylmagnesium bromide, with the simplified formula , is a magnesium-containing organometallic compound. It is commercially available as a solution in diethyl ether or tetrahydrofuran (THF). Phenylmagnesium bromide is a Grignard reagent. It is o ...

and

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

: :PhMgBr + Se → PhSeMgBr :PhSeMgBr + HCl → PhSeH + MgBrCl Since benzeneselenol does not have a long shelflife, it is often generated in situ. A common method is by reduction of

diphenyldiselenide Diphenyl diselenide is the chemical compound with the formula (C6H5)2Se2, abbreviated Ph2Se2. This orange-coloured solid is the oxidized derivative of benzeneselenol. It is used as a source of the PhSe unit in organic synthesis. Preparation ...

. A further reason for this conversion is that often, it is the anion that is sought.

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 The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical s ...

of which is estimated to be between 67 and 74 kcal/mol. In contrast, the S-H BDE for thiophenol is near 80 kcal/mol. The product is

diphenyl diselenide Diphenyl diselenide is the chemical compound with the formula (C6H5)2Se2, abbreviated Ph2Se2. This orange-coloured solid is the oxidized derivative of benzeneselenol. It is used as a source of the PhSe unit in organic synthesis. Preparation a ...

as shown in this idealized equation: : The presence of the

diselenide Diselenide may refer to: *Diselane, H-Se-Se-H *Carbon diselenide, CSe2, a yellow-orange oily liquid with pungent odor * Any organic chemical compound with a selenium-selenium bond, R-Se-Se-R **Diphenyl diselenide, (C6H5)–Se–Se–(C6H5) *Metal ...

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: : PhSeH → PhSe⁻ + H⁺ It is approximately seven times more acidic than the related

thiophenol Thiophenol is an organosulfur compound with the formula C6H5SH, sometimes abbreviated as PhSH. This foul-smelling colorless liquid is the simplest aromatic thiol. The chemical structures of thiophenol and its derivatives are analogous to phenols ...

. Both compounds dissolve in water upon the addition of base. The conjugate base is PhSe⁻, a potent

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

History

Benzeneselenol was first reported in 1888 by the reaction of

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...

with

selenium tetrachloride Selenium tetrachloride is the inorganic compound composed with the formula SeCl4. This compound exists as yellow to white volatile solid. It is one of two commonly available selenium chlorides, the other example being selenium monochloride, Se2 ...

(SeCl₄) in the presence of

aluminium trichloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often contam ...

(AlCl₃).

Safety

The compound is intensely malodorous and, like other organoselenium compounds, toxic.

References

{{reflist Organoselenium compounds Selenols Phenyl compounds Foul-smelling chemicals