Benzeneselenol

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.^[1]

Benzeneselenol

Names
IUPAC name Benzeneselenol
Other names Phenylselenol Selenaphenol Selenophenol
Identifiers
CAS Number	645-96-5 Y
3D model (JSmol)	Interactive image
ChemSpider	62734 Y
ECHA InfoCard	100.010.417
EC Number	211-457-2
PubChem CID	69530
CompTox Dashboard (EPA)	DTXSID30275709
InChI InChI=1S/C6H6Se/c7-6-4-2-1-3-5-6/h1-5,7H Y Key: WDODWFPDZYSKIA-UHFFFAOYSA-N Y InChI=1/C6H6Se/c7-6-4-2-1-3-5-6/h1-5,7H Key: WDODWFPDZYSKIA-UHFFFAOYAD
SMILES [SeH]c1ccccc1
Properties
Chemical formula	C6H5SeH
Molar mass	157.085 g·mol−1
Appearance	colorless liquid
Odor	extremely foul yet characteristic
Density	1.479 g/cm3
Boiling point	71 to 72 °C (160 to 162 °F; 344 to 345 K) (18 mmHg)
Solubility in water	slightly
Solubility in other solvents	most organic solvents
Refractive index (nD)	1.616
Structure
Dipole moment	1.1 D
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	toxic
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H301, H331, H373, H410
Related compounds
Related compounds	Phenol Thiophenol Hydrogen selenide Diphenyl diselenide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

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:

4 PhSeH + O₂ → 2 PhSeSePh + 2 H₂O

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 pK_a 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. Both compounds dissolve in water upon the addition of base. The conjugate base is PhSe⁻, a potent nucleophile.^[1]

History

Benzeneselenol was first reported in 1888 by the reaction of benzene with selenium tetrachloride (SeCl₄) in the presence of aluminium trichloride (AlCl₃).^[4][5]

Safety

The compound is intensely malodorous^[6] and, like other organoselenium compounds, toxic.

References

1. Sonoda, Noboru; Ogawa, Akiya; Recupero, Francesco (2005). "Benzeneselenol". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rb018.pub2. ISBN 0471936235.
2. Foster, D. G. (1944). "Selenophenol". Organic Syntheses. 24: 89. doi:10.15227/orgsyn.024.0089.
3. Chandra, Asit K.; Nam, Pham-Cam; Nguyen, Minh Tho (2003). "The S−H Bond Dissociation Enthalpies and Acidities of Para and Meta Substituted Thiophenols: A Quantum Chemical Study". The Journal of Physical Chemistry A. 107 (43): 9182–9188. Bibcode:2003JPCA..107.9182C. doi:10.1021/jp035622w.
4. Chabrié, M. C. (1888). "Premiers essais de synthèse de composés organiques séléniés dans la série aromatique". Bulletin de la Société Chimique de Paris. 50: 133–137.
5. Chabrié, M. C. (1890). "Sur la synthèse de quelques composés séléniés dans la série aromatique". Annales de Chimie et de Physique. 6 (20): 202–286.
6. Lowe, D. (2012-05-15). "Things I wont work with: Selenophenol". In the Pipeline.