Thiophenols

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.

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. 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).

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

Thiophenol can be manufactured from chlorobenzene and hydrogen sulfide over alumina at . The disulfide is the primary byproduct. The reaction medium is corrosive and requires ceramic or similar reactor lining. Aryl iodides and sulfur in certain conditions may also produce thiophenols.

Applications

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

Properties and reactions

Acidity

Thiophenol has appreciably greater acidity than does phenol, as is shown by their p''K''_a 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. 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).

Coordination to metals

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

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.CDC - NIOSH Pocket Guide to Chemical Hazards
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References

External links

Thiophenol
Toxicology Data Network

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Thiols
Phenyl compounds
Foul-smelling chemicals