Submitted by F. B. Dains, R. Q. Brewster, and C. P. Olander.
Checked by C. S. Marvel and L. T. Sandborn.
1. Procedure
In a
500-cc. round-bottomed flask (Note
1), fitted with a
mechanical stirrer and surrounded by an
ice-salt cooling bath, are placed
54 g. (43 cc., 0.71 mole) of carbon disulfide and
90 cc. (1.3 moles) of concentrated aqueous ammonia (sp. gr. 0.9). The stirrer is started, and
56 g. (0.6 mole) of aniline (Note
2) is run into the mixture from a
separatory funnel at such a rate that the addition is complete in about twenty minutes. The stirring is continued for thirty minutes after all the
aniline has been added, and then the reaction mixture is allowed to stand for another thirty minutes. During this time a heavy precipitate of
ammonium phenyldithiocarbamate separates and may even stop the stirrer.
The salt is dissolved in 800 cc. of water (Note
3) and transferred to a
5-l. round-bottomed flask. To the solution is added with constant stirring a solution of
200 g. (0.6 mole) of lead nitrate in 400 cc. of water.
Lead sulfide separates as a heavy brown precipitate which soon turns black. The mixture is then distilled with steam into a
receiver containing
5–10 cc. of 1 N sulfuric acid as long as any oil comes over (Note
4). About 2–3 l. of distillate is collected. The product is separated from the water and weighs
63–66 g.
The oil is dried over a little
calcium chloride and distilled under reduced pressure. The yield of
phenyl isothiocyanate boiling at
120–121°/35 mm. is
60–63 g. (
74–78 per cent of the theoretical amount) (Note
5) and (Note
6).
2. Notes
1. If the reaction is carried out in a
beaker, so much
ammonia is lost by volatilization that the crystalline
ammonium phenyldithiocarbamate is not formed. The temperature should be from 0° to 10° to avoid loss of
ammonia.
2. Ordinary technical
aniline was used in these experiments.
3. The transfer of the salt to the 5-l. flask is conveniently made by the addition of four successive 200-cc. portions of water to the flask containing the salt.
5. Larger runs give somewhat lower percentage yields; thus
280 g. of aniline gives about
250 g. (
61 per cent of the theoretical amount) of redistilled
phenyl isothiocyanate.
6. The following slightly modified procedure is generally applicable to the preparation of aryl isothiocyanates.
To a solution of
90 g. (0.50 mole) of p-bromoaniline,
60 g. (0.78 mole) of carbon disulfide and
90 cc. of 95 per cent alcohol at 10–15° is added
81.6 g. (0.63 mole) of concentrated aqueous ammonia. The milky suspension, in a
stoppered flask covered with a towel, is shaken occasionally until a clear solution is obtained. Considerable heat is evolved, and crystals of the intermediate
dithiocarbamate soon separate. After standing overnight the crystals are filtered, washed with
ether, dissolved in 3 l. of cold water, and stirred while a solution of
174 g. (0.5 mole) of lead nitrate is slowly added. Stirring is continued for fifteen to twenty minutes, and then the
p-bromophenylisothiocyanate is steam-distilled into a flask acidified with
sulfuric acid. The product, which solidifies, is separated and dried. It weighs
30.5 g. (a yield of
27 per cent) and melts at
60–61°.
In this procedure the use of alcohol is essential; without it no isothiocyanate is obtained. It is also advisable with very reactive aromatic amines to add the
ammonia in small portions so as to avoid a too vigorous reaction and to keep the temperature of the contents of the stoppered flask below 35°. With some substituted anilines the intermediate
dithiocarbamate does not crystallize although the heat evolved indicates that a reaction has taken place. When this happens the solution can be diluted with water and treated with
lead nitrate, but the yield will usually be small.
3. Discussion
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