Org. Synth. 1951, 31, 105
DOI: 10.15227/orgsyn.031.0105
THIOLACETIC ACID
[Acetic acid, thiol-]
Submitted by E. K. Ellingboe
1
Checked by Arthur C. Cope and Malcolm Chamberlain.
1. Procedure
Caution! All the steps of this procedure should be carried out under a hood because of the highly toxic nature of hydrogen sulfide and the probable toxicity and persistent unpleasant odor of thiolacetic acid.
A 200-ml. three-necked flask is fitted with a mercury-sealed glass stirrer, a reflux condenser, and a gas inlet tube and thermometer, both of which extend into the lower half of the flask. The top of the condenser is connected to a mercury bubbler tube, and the gas inlet tube is attached to the inlet tube of a gas-washing bottle which serves as a safety trap to prevent liquid from being drawn into the hydrogen sulfide source. The gas-washing bottle is connected through a drying tube containing anhydrous calcium sulfate (Drierite) to a T-tube. The vertical arm of the T-tube dips into mercury and forms a safety valve; the other arm is connected to a commercial cylinder (Note 1) or other source of hydrogen sulfide. To the flask are added 107 g. (100 ml., 1 mole) of 95% acetic anhydride and 1 g. (0.025 mole) of powdered sodium hydroxide. The assembly of the flask, condenser, inlet tube, and thermometer is weighed and arranged so that the amount of hydrogen sulfide introduced can be determined by subsequent weighing. The stirrer is started, and hydrogen sulfide is passed into the mixture as rapidly as possible without much loss of the gas through the bubbler connected to the top of the condenser. The temperature of the mixture rises to 55° within 30 minutes and is kept at 50–55° by intermittent cooling. The temperature begins to drop after 14–17 g. of hydrogen sulfide has been absorbed and is maintained at 50–55° by external heating. After a total reaction period of 6 hours, hydrogen sulfide ceases to be absorbed and the gain in weight amounts to about 31 g.
The reaction mixture is transferred to a
250-ml. Claisen flask (Note 2) and distilled rapidly at 200 mm. in order to separate the sodium salts
(Note 3). The distillate of
thiolacetic acid and
acetic acid, b.p. 35–82°/200 mm., amounts to 120–124 g. It is fractionally distilled at atmospheric pressure through an efficient, variable take-off type
column2 with a
30- to 40-cm. section packed with glass helices. The fraction boiling at
86–88°,
n25D 1.4612, is nearly pure
thiolacetic acid and amounts to
55–57.5 g. (
72–76%) based on the
acetic anhydride (Note 4) and
(Note 5). The residual liquid is mainly
acetic acid. If distillation is continued after separation of the
thiolacetic acid, the vapor temperature rises rapidly to the boiling point of
acetic acid.
2. Notes
1.
Cylinders of
hydrogen sulfide are available from the Matheson Company, Inc., East Rutherford, New Jersey.2.
An all-glass distillation assembly equipped with ground joints is advisable because the hot vapor of
thiolacetic acid rapidly softens
rubber stoppers.
3.
The product decomposes excessively if fractionation is attempted in the presence of the sodium salts. In larger-scale preparations the initial distillation should be conducted in small batches or, preferably, in a continuous stripping still.
4.
Refractionation yields pure
thiolacetic acid (with about 10% loss) with the following physical constants: b.p.
87°/760 mm.,
50°/200 mm.,
34°/100 mm.;
n25D 1.4630;
d254 1.0634. Higher boiling points at atmospheric pressure have been reported:
88–91°,
3 89°,
4 and 93°.
55.
The submitter has prepared
thiolacetic acid in yields of
65–70 g. (
85–92%) by a similar procedure in which the reaction mixture is placed in the glass bottle (provided with a heating jacket and thermometer or thermocouple) of a low-pressure hydrogenation apparatus.
6 The mixture is shaken and
hydrogen sulfide is introduced at 25–35 p.s.i., with repressuring to 35–40 p.s.i. whenever the pressure drops to 10 p.s.i. The heat of reaction raises the temperature to 60–65° in 12–15 minutes, after which the internal temperature is maintained at 60° by heating.
Hydrogen sulfide absorption becomes negligible after 4 hours, and the mixture is allowed to cool while under pressure, vented, and the product is isolated in the manner described. The reaction also was conducted in a steel autoclave or hydrogenation bomb at the full pressure of
hydrogen sulfide in a commercial cylinder (about 300 p.s.i. at room temperature).
Hydrogen sulfide poisons the ordinary hydrogenation catalysts, and low-pressure cylinders or hydrogenation bombs exposed to
hydrogen sulfide may not be suitable for subsequent use in catalytic hydrogenations.
3. Discussion
Thiolacetic acid has been prepared from
acetic acid and
phosphorus pentasulfide;
5 from
acetyl chloride and
potassium hydrosulfide;
7 by the hydrolysis of
diacetyl sulfide;
8 from
acetic anhydride and
hydrogen sulfide;
3,4,9,10 and from
acetyl chloride and
hydrogen sulfide in the presence of
aluminum chloride.
11 The procedure described
9 differs from other procedures employing
acetic anhydride and
hydrogen sulfide3,4 most importantly in the use of alkaline rather than acidic catalysts, which the submitter found to cause slower absorption of
hydrogen sulfide under pressure and to yield considerable
diacetyl sulfide in addition to
thiolacetic acid. Other alkaline catalysts which have been used to effect the reaction between
acetic anhydride and
hydrogen sulfide are
triethylamine12 and
pyridine.
13
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
calcium sulfate (Drierite)
acetic acid (64-19-7)
acetic anhydride (108-24-7)
sodium hydroxide (1310-73-2)
acetyl chloride (75-36-5)
hydrogen sulfide (7783-06-4)
mercury (7439-97-6)
aluminum chloride (3495-54-3)
pyridine (110-86-1)
potassium hydrosulfide (1310-61-8)
phosphorus pentasulfide (1314-80-3)
triethylamine (121-44-8)
Thiolacetic acid
Acetic acid, thiol- (1918-77-0)
diacetyl sulfide (3232-39-1)
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