Checked by James Cason and Ralph J. Fessenden.
1. Procedure
Since
carbon disulfide is highly flammable and
hydrogen sulfide highly toxic, this reaction should be carried out in a
hood, with due precaution against fire. It is also recommended that
tetracyanoethylene not be allowed to come into contact with the skin.
A
1-l. three-necked flask is fitted with a
sealed mechanical stirrer, a
condenser protected by a drying tube, a
thermometer, and an
inlet tube extending to the bottom of the flask. A solution of
25.6 g. (0.2 mole) of recrystallized tetracyanoethylene (p. 877) in
300 ml. of acetone is placed in the flask, and
300 ml. of carbon disulfide is added. The flask and its contents are cooled to 0° by means of a
salt-ice bath. With good stirring,
hydrogen sulfide is passed into the reaction mixture at a moderate rate while the temperature is maintained at 0–5°. The solution becomes milky after a few minutes owing to the formation of colloidal
sulfur. The
hydrogen sulfide addition is continued for about 30 minutes, or until the solution is thoroughly saturated.
The
hydrogen sulfide addition is temporarily suspended, and
100 ml. of pyridine is added rapidly in one portion through the condenser as the solution is stirred vigorously (Note
1). The solution becomes clear, and then
2,5-diamino-3,4-dicyanothiophene begins to precipitate immediately. The
hydrogen sulfide addition is resumed and is continued for about 30 minutes while the temperature of the reaction mixture is maintained at 0–5°. Finally, the reaction mixture is stirred for an additional 30 minutes at 0–5°, then the yellow precipitate of the thiophene is collected on a
Büchner funnel, thoroughly washed with about
500 ml. of acetone, and dried in the air or in a
vacuum desiccator. The yield of crude product of yellow or buff color amounts to
30–31 g. (
92–95%).
This material is sufficiently pure for most purposes. If a purer product is desired, the crude material is dissolved in
300 ml. of dimethylformamide,
10 g. of activated alumina (48–100 mesh) is added, and the mixture is filtered. The filtrate is heated to 80–90° on a
steam bath, then 1 l. of boiling water is added immediately (Note
2). The resultant mixture is cooled in an
ice bath, and the light buff crystals of
2,5-diamino-3,4-dicyanothiophene that separate are collected on a Büchner funnel and thoroughly washed with
500 ml. of acetone; weight
26–28 g. (
79–85%). The product has no definite melting point but sublimes with some decomposition when heated above 250°.
2. Notes
1. Unless the
pyridine is added quite rapidly, some of the product will begin to precipitate before all of the
sulfur has dissolved, and the final product will be contaminated with
sulfur.
2. This operation should be carried out as rapidly as possible, since prolonged heating in
dimethylformamide results in loss of product.
3. Discussion
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