Thiophenol

Thiophenol is a chemical compound with the formula C₆H₅SH, and sometimes abbreviated as PhSH. The foul-smelling liquid is the principal aromatic thiol. The chemical structures of thiophenols 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, thio- commonly means that the 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 medicine merthiolate is a thiophenol.

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Synthesis

Many ways exist to generate thiophenol and related compounds, although thiophenol is usually purchased for laboratory operations.

• Reduction of benzenesulfonyl chloride with zinc.^[1]
• Action of elemental sulfur on phenyl magnesium halide or phenyllithium followed by acificiation.

Properties

Acidity

Thiophenol has appreciably greater acidity than does phenol. A similar pattern is seen for H₂S vs. H₂O and all thiols vs. the corresponding alcohols. Treatment of PhSH with strong base such as sodium hydroxide (NaOH) or sodium metal affords the salt sodium thiophenolate (PhSNa). Such salts are readily oxidized.

Alkylation

The thiophenolate is highly nucleophilic, which translates to an high rate of alkylation.^[2] Thus, treatment of C₆H₅SH with methyl iodide in the presence of a base gives methyl phenyl sulfide, C₆H₅SCH₃, a thioether. Such reactions are fairly irreversible. C₆H₅SH also adds to α,β-unsaturated carbonyls.

Oxidation

A substantial difference between sulfur and oxygen is that thiophenolate is oxidized to diphenyl disulfide:

2 C₆H₅SH + 1/2 O₂ → C₆H₅S-SC₆H₅ + 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. Of course free H atoms are almost never observed in solution, but C₆H₅SH delivers the equivalent result by adding H atoms to various substrates.

Chlorination

Phenylsulfenyl chloride, a blood-red liquid (b.p. 41–42 °C), can be prepared by the reaction of thiophenol with chlorine (Cl₂).^[3]

Coordination to metals

Metal cations form thiophenolates, some of which are polymeric. One example is "C₆H₅SCu," obtained by treating copper(I) chloride with thiophenol.^[4]

Safety

Thiophenol is highly toxic.

References

1. ^ Adams, R.; C. S. Marvel, C. S.. "Thiophenol". Org. Synth.; Coll. Vol. 1: 504. 
2. ^ Campopiano, O. "Thiophenol" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
3. ^ Barrett, A. G. M.; Dhanak, D.; Graboski, G. G.; Taylor, S. J. (1993). "(Phenylthio)nitromethane". Org. Synth.; Coll. Vol. 8: 550. 
4. ^ 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". Org. Synth.; Coll. Vol. 6: 248.