A solution of
200 g. (3 moles) of potassium hydroxide (85%) pellets in
800 ml. of 90% ethanol (Note
1) is prepared with mechanical stirring in a
2-l. three-necked round-bottomed flask. The flask is fitted with a
500-ml. dropping funnel and a
gas inlet tube extending to the bottom of the flask, and
hydrogen sulfide is passed in through the inlet tube with stirring and cooling until the solution is saturated and does not give an alkaline reaction with
phenolphthalein (Note
2) and (Note
3). The mixture is further cooled to 10–15° by means of an
ice bath, and
200 g. (1.41 moles) of freshly distilled benzoyl chloride (Note
4) is introduced drop-wise over a period of about 1.5 hours with stirring while the temperature is kept below 15°. After the addition of the
benzoyl chloride has been completed, the reaction mixture is stirred for an additional hour. The
potassium chloride which precipitates during the addition is separated quickly by filtration through a
Büchner funnel and is washed with about
200 ml. of 95% ethanol. The filtrate is placed in a 2-l. round-bottomed flask fitted with a condenser arranged for distillation and evaporated to dryness under reduced pressure on a
steam bath (Note
5). The solid residue, consisting mainly of
potassium thiobenzoate, is dissolved in about 700 ml. of cold water (Note
6), and the solution is filtered if considerable insoluble material is present (Note
4). The alkaline solution is extracted with
500 ml. of benzene in order to remove any neutral material. The aqueous layer is then acidified with cold
6N hydrochloric acid (Note
6) and extracted with two
500-ml. portions of peroxide-free ether (Note
7). The
ether layer is washed with several portions of cold water and dried over anhydrous
sodium sulfate. The
ether is evaporated under reduced pressure on a steam bath (Note
5), and the residue is fractionated immediately through a
short (15–20 cm.) Vigreux column at reduced pressure, dry
nitrogen being admitted through the capillary (Note
8). After a very small fore-run, the yellow-orange
thiobenzoic acid distils at
85–87°/10 mm. (
95–97°/15 mm.); yield
120–150 g. (
61–76%);
nD20 1.6027. Upon refractionation, the light-yellow
thiobenzoic acid, as determined by titration with standard base or alcoholic
iodine, is about 99.5% pure;
nD20 1.6030.