Org. Synth. 1921, 1, 71
DOI: 10.15227/orgsyn.001.0071
THIOPHENOL
[Benzenethiol]
Submitted by Roger Adams and C. S. Marvel.
Checked by H. T. Clarke and Katharine Ogden.
1. Procedure
In a
12-l. round-bottomed flask are placed 7.2 kg. of cracked ice and
2.4 kg. (1300 cc., 23.4 moles) of concentrated sulfuric acid (sp. gr. 1.84). The mixture becomes very cold and is kept at −5° to 0° by means of an
ice-salt bath. Mechanical stirring is started and
600 g. (3.4 moles) of crude benzenesulfonyl chloride (p. 84) is gradually introduced (one-half hour)
(Note 1). Then
1.2 kg. (16.5 atoms) of zinc dust (90 per cent) is added
(Note 2) in portions as rapidly as possible without allowing the temperature to rise above that mentioned (this requires about one-half hour)
(Note 3). The contents of the flask are stirred for one to one and one-half hours longer, the temperature being kept at 0° or below during the whole period. A
stopper holding an
efficient reflux condenser (lower end not constricted) and stirrer is now attached; the ice bath is removed
(Note 4); and the reaction mixture is allowed to warm up spontaneously or it may be warmed up with a low flame, the stirring being continued. Within a very few (two to five) minutes, in many runs, a rather vigorous reaction with the evolution of much
hydrogen takes place. Under these circumstances it is advisable to cool the flask momentarily by a stream of water. The reflux condenser will, however, generally take care of the vapors satisfactorily.
After the first ebullition has subsided, no further attention is required, and the mixture is heated to boiling over a ring burner until the solution becomes clear. The stirring should be continued during the refluxing, and the stirrer should be arranged so as to suck the liquid downward from the surface. This helps distribute the zinc which holds hydrogen and floats on top of the liquid. About four to seven hours are required for the heating (Note 5). The thiophenol is then distilled with steam, this process requiring about one hour. The product is separated from the water, dried with calcium chloride (shaking with calcium chloride for three to five minutes is generally sufficient), and distilled. The crude material weighs 359 g. (96 per cent of the theoretical amount), and the pure compound boiling at 166–169° (71°/15mm.) weighs 340 g. (91 per cent of the theoretical amount). Ten to twelve hours are required for the whole experiment up to the final distillation.
2. Notes
1.
Since
benzenesulfonyl chloride solidifies at 15°, it is necessary to add it to the
sulfuric acid slowly, with stirring, so that it will be as finely divided as possible when the
zinc is added.
2.
The directions given in the literature advise the addition of the
zinc dust to the
sulfuric acid and then the addition of the
benzenesulfonyl chloride. This procedure has been reversed in the way described above so that the minimum amount of
hydrogen will be lost during the addition of the
zinc.
3.
It is essential that the temperature during the first part of the reaction be kept at 0° or below, for otherwise poor yields are obtained. In order to hold the temperature at 0°, it is absolutely essential to add the
zinc dust at least as slowly as described.
4.
After the initial reaction at a low temperature had taken place, it was, in several instances, found convenient to allow the reaction mixture to stand overnight before proceeding further. The flask was thus left in the ice, which gradually melted, allowing the reaction mixture to warm up to room temperature. Under these conditions, however, a poor yield generally resulted. Upon closer study, it was discovered that the small amount of product obtained was due to the sudden vigorous reaction, with consequent loss through the condenser, which took place when a certain temperature was reached. The reaction mixture may be safely allowed to stand only if a very efficient condenser is used. In the checking of this experiment, no sudden reaction was observed, and heating with a small flame after the initial mixing and stirring at 0° gave excellent results. The difference in the experiments may have been due to a slightly different grade of raw materials.
5.
Vigorous stirring while the reaction mixture is heated reduces materially the time of the reduction (from twelve to seven hours or less), as it brings the
zinc, which otherwise floats, into intimate contact with the acid and
sulfonyl chloride.
3. Discussion
Two general methods are available for the preparation of thiophenols. The first is by the treatment of
aryldiazonium chlorides with
potassium ethyl xanthogenate and then heating with alkali to give thiophenols.
1 This reaction has been used for the preparation of a large number of thiophenols, but it suffers from the disadvantage of having many side-reactions and in the danger of occasional violent explosions
2 when conditions are not correct. The second method involving the reduction of sulfonyl chlorides by
zinc dust and
sulfuric acid,
3 although limited to sulfonyl chlorides not having another group such as nitro which might be reduced, gives better yields of pure product than the
xanthogenate process and avoids
ether extraction and treatment with alcoholic
potassium hydroxide.
Thiophenol itself has also been prepared from
diphenyl sulfide4 and from
chlorobenzene5 by heating with aqueous
sodium sulfide under pressure.
This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
sulfonyl chloride
calcium chloride (10043-52-4)
sulfuric acid (7664-93-9)
ether (60-29-7)
hydrogen (1333-74-0)
Benzenesulfonyl chloride (98-09-9)
chlorobenzene (108-90-7)
potassium hydroxide (1310-58-3)
zinc (7440-66-6)
sodium sulfide (1313-82-2)
Thiophenol,
Benzenethiol (108-98-5)
potassium ethyl xanthogenate (140-89-6)
xanthogenate
Diphenyl sulfide (139-66-2)
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