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 except the oxygen atom in the hydroxyl group (-OH) bonded to the aromatic ring is replaced by a sulfur atom. The prefix thio- implies a sulfur-containing compound and when used before a root word name for a compound which would normally contain an oxygen atom, in the case of 'thiol' that the alcohol oxygen atom is replaced by a sulfur atom.

Not to be confused with benzyl mercaptan.
Thiophenol
Skeletal formula
Ball-and-stick model
Names
Preferred IUPAC name
Benzenethiol[1]
Other names
  • Thiophenol (or phenylthiol)
  • Phenyl mercaptan
  • Mercaptobenzene
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.003.306
RTECS number
  • DC0525000
UNII
Properties
C6H6S
Molar mass 110.17 g·mol−1
Appearance Colorless liquid
Odor Unpleasant, pungent
Density 1.0766 g/mL
Melting point −15 °C (5 °F; 258 K)
Boiling point 169 °C (336 °F; 442 K)
0.08%[2]
Solubility Most organic solvents; aqueous base
Vapor pressure 1 mmHg (18°C)[2]
Acidity (pKa)
-70.8·10−6 cm3/mol
Hazards
Main hazards Toxic
R-phrases (outdated) R10 R24/25 R26 R41
S-phrases (outdated) S23 S26 S28 S36/37/39 S45
NFPA 704 (fire diamond)
Flash point 56 °C; 132 °F; 329 K[2]
NIOSH (US health exposure limits):
PEL (Permissible)
 none[2]
REL (Recommended)
 C 0.1 ppm (0.5 mg/m3) [15-minute][2]
IDLH (Immediate danger)
 N.D.[2]
Related compounds
Related thiols
 1,2-Benzenedithiol
Benzenemethanethiol
Related compounds
 Phenol
Benzeneselenol
Diphenyl disulfide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Thiophenols also describes a class of compounds formally derived from thiophenol itself. All have a sulfhydryl group (-SH) covalently bonded to an aromatic ring. The organosulfur ligand in the medicine thiomersal is a thiophenol.

Synthesis

There are several methods of synthesis for thiophenol and related compounds, although thiophenol itself is usually purchased for laboratory operations. Methods are the reduction of benzenesulfonyl chloride with zinc.[5] and the action of elemental sulfur on phenyl magnesium halide or phenyllithium followed by acidification.

Via the Newman–Kwart rearrangement, phenols (1) can be converted to the thiophenols (5) by conversion to the O-aryl dialkylthiocarbamates (3), followed by heating to give the isomeric S-aryl derivative (4).[6]

In the Leuckart thiophenol reaction, the starting material is an aniline through the diazonium salt (ArN2X) and the xanthate (ArS(C=S)OR).[7][8] Alternatively, sodium sulfide and triazene can react in organic solutions and yield thiophenols.[9]

Thiophenol can be manufactured from chlorobenzene and hydrogen sulfide over alumina at 700 to 1,300 °F (371 to 704 °C). The disulfide is the primary byproduct.[10] The reaction medium is corrosive and requires ceramic or similar reactor lining. Aryl iodides and sulfur in certain conditions may also produce thiophenols.[11]

Applications

Thiophenols are used in the production of pharmaceuticals including of sulfonamides. The antifungal agents butoconazole merthiolate are derivatives of thiophenols.[9]

Properties and reactions

Acidity

Thiophenol has appreciably greater acidity than does phenol, as is shown by their pKa values (6.62 for thiophenol and 9.95 for phenol). A similar pattern is seen for H2S versus H2O, and all thiols versus the corresponding alcohols. Treatment of PhSH with strong base such as sodium hydroxide (NaOH) or sodium metal affords the salt sodium thiophenolate (PhSNa).

Alkylation

The thiophenolate is highly nucleophilic, which translates to a high rate of alkylation.[12] Thus, treatment of C6H5SH with methyl iodide in the presence of a base gives methyl phenyl sulfide, C6H5SCH3, a thioether often referred to as thioanisole. Such reactions are fairly irreversible. C6H5SH also adds to α,β-unsaturated carbonyls via Michael addition.

Oxidation

Thiophenols, especially in the presence of base are easily oxidized to diphenyl disulfide:

4 C6H5SH + O2 → 2 C6H5S-SC6H5 + 2 H2O

The disulfide can be reduced back the thiol using sodium borohydride followed by acidification. This redox reaction is also exploited in the use of C6H5SH as a source of H atoms.

Chlorination

Phenylsulfenyl chloride, a blood-red liquid (b.p. 41–42 °C, 1.5 mm Hg), can be prepared by the reaction of thiophenol with chlorine (Cl2).[13]

Coordination to metals

Metal cations form thiophenolates, some of which are polymeric. One example is "C6H5SCu," obtained by treating copper(I) chloride with thiophenol.[14]

Safety

The US National Institute for Occupational Safety and Health has established a recommended exposure limit at a ceiling of 0.1 ppm (0.5 mg m−3), and exposures not greater than 15 minutes.[15]

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References
  1. "Front Matter". Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. pp. P001–P004. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
  2. NIOSH Pocket Guide to Chemical Hazards. "#0050". National Institute for Occupational Safety and Health (NIOSH).
  3. Cox, Brian G. Acids and Bases: Solvent Effects on Acid-base Strength. 1st ed. Oxford, UK: Oxford UP, 2013.
  4. Bordwell, Frederick G.; Hughes, David L. (1982). "Thiol acidities and thiolate ion reactivities toward butyl chloride in dimethyl sulfoxide solution. The question of curvature in Broensted plots". The Journal of Organic Chemistry. 47 (17): 3224–3232. doi:10.1021/jo00138a005.
  5. Adams, R.; C. S. Marvel, C. S. "Thiophenol". Organic Syntheses.CS1 maint: multiple names: authors list (link); Collective Volume, 1, p. 504.
  6. Melvin S. Newman and Frederick W. Hetzel (1971). "Thiophenols from Phenols: 2-Naphthalenethiol". Organic Syntheses. 51: 139. doi:10.15227/orgsyn.051.0139.
  7. Leuckart, J. prakt. Chem., [2] 41, 189 (1890).
  8. Tarbell, D. S.; Fukushima, D. K. (1947). "m-Thiocresol". Organic Syntheses. 27: 81. doi:10.15227/orgsyn.027.0081.; Collective Volume, 3, p. 809
  9. Kazem-Rostami, Masoud; Khazaei, Ardeshir; Moosavi-Zare, Ahmad; Bayat, Mohammad; Saednia, Shahnaz (2012). "One-pot synthesis of thiophenols". Synlett. 23 (13): 1893–1896. doi:10.1055/s-0032-1316557.
  10. US Patent 2,490,257, Duncan J. Crowley & Alvin L Kosak, "Mono- and Polyalkyl Mono- and Polynuclear Mercaptans", issued 1949-12-06, assigned to Socony-Vacuum Oil Co.
  11. Jiang, Y.; Qin, Y.; Xie, S.; Zhang, X.; Dong, J.; Ma, D., A (2009). "General and Efficient Approach to Aryl Thiols: CuI-Catalyzed Coupling of Aryl Iodides with Sulfur and Subsequent Reduction". Organic Letters. 22: 52. PMID 31833778.CS1 maint: multiple names: authors list (link)
  12. Campopiano, O. (2004). "Thiophenol". Encyclopedia of Reagents for Organic Synthesis. New York: John Wiley & Sons. doi:10.1002/047084289 (inactive 2020-01-22)..
  13. Barrett, A. G. M.; Dhanak, D.; Graboski, G. G.; Taylor, S. J. (1993). "(Phenylthio)nitromethane". Organic Syntheses.CS1 maint: multiple names: authors list (link); Collective Volume, 8, p. 550
  14. Posner, G. H.; Whitten, C. E. "Secondary and Tertiary Alkyl Ketones from Carboxylic Acid Chlorides and Lithium Phenylthio(alkyl)cuprate Reagents: tert-Butyl Phenyl Ketone". Organic Syntheses.CS1 maint: multiple names: authors list (link); Collective Volume, 6, p. 248
  15. CDC - NIOSH Pocket Guide to Chemical Hazards
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