Checked by William S. Johnson and William T. Tsatsos.
1. Procedure
A.
Cyanamide. In a
large mortar are placed
57 g. (54 ml., 0.75 mole) (Note 1) of glacial acetic acid and 135 ml. of water (Note
2). To this solution,
40 g. (0.5 mole) of calcium cyanamide (Note
3) is added slowly (Note
4) with good stirring and grinding. As the introduction of the
calcium cyanamide proceeds, small quantities of
acetylene are evolved, while the initially thin cream gradually turns into a thick, dark-gray to black paste. The mixture must remain acidic to litmus throughout the addition. After being dried at 40–50° for 12–18 hours in a
vacuum oven, at a pressure of 30 mm. or less, the material is obtained as a pale-gray, dry powder. This is extracted exhaustively in a Soxhlet apparatus, for 2- to 3-hour periods, with two successive
400-ml. portions of ether (Note 5) containing a few drops of dilute acetic acid. The ethereal extracts (Note
6) are each dried over
30-g. portions of anhydrous sodium sulfate and combined, and the solvent is removed under reduced pressure (Note
7). The colorless, viscous, oily residue of
cyanamide is suitable for use in the next stage. The yield of
cyanamide is
10.5–15.8 g. (
50–75% calculated on the basis of the formula CaNCN) (Note
8) and (Note
9).
The material may be crystallized from boiling
methanol (1 ml. per gram of crude material) and forms lustrous, colorless, thick, prismatic needles, which melt with decomposition (Note
11) and (Note
12). They are separated by suction filtration at 0° (Note
13) and quickly rinsed with a very little ice-cold
methanol. The product is hygroscopic and is quickly pressed between filter paper, then dried in a vacuum desiccator over phosphorus pentoxide. The filtrates yield a further crop on partial evaporation under reduced pressure, the total recovery of recrystallized material being
85–90% (Note
14).
2. Notes
1. The exact amount of
acetic acid required by a particular sample of
calcium cyanamide is first determined volumetrically as follows: A weighed sample of
calcium cyanamide (approximately 1 g.) is suspended in about 50 ml. of distilled water and titrated with standard
hydrochloric acid (preferably of approximately normal strength), using
phenolphthalein as indicator. Acid is added until the pink color of the indicator does not reappear within 2–3 minutes. From the results of the titration the amount of
acetic acid required is calculated by proportion, a 10% excess being allowed to ensure that the reaction mixture remains acid throughout the experiment.
2. In order to facilitate drying at a later stage, as little water is used as will produce a paste that can still be effectively mixed.
3. Commercial
calcium cyanamide (nitrolime), containing
carbon and small quantities of
calcium carbide, is suitable for this preparation.
4. The addition of the
calcium cyanamide to the acid should be slow enough to ensure thorough mixing and to prevent the reaction mixture from becoming hot.
5. The checkers found it desirable to saturate the
ether (by shaking) with water before extraction. The Soxhlet extraction should not be prolonged for more than 3–4 hours. If a longer extraction period is employed, a fresh portion of
ether should be used so that the extracted material will not be subjected to heat for too long a period.
6.
Cyanamide may be kept unchanged at 0° in ethereal solution in the presence of traces of
acetic acid. The ethereal extracts from several runs of
calcium cyanamide may therefore be combined and worked up collectively.
7. Distillation is best carried out from a previously weighed
small flask, and the weighed residue of
cyanamide is immediately dissolved in
methanol for the next stage.
8. The yields of
cyanamide from commercial
calcium cyanamide vary from sample to sample but do not fluctuate greatly for one particular specimen. The present procedure was found to be satisfactory for preparing
15- to 30-g. batches of
cyanamide.
9. If desired, the
cyanamide may be crystallized from a mixture of
ether and
benzene; however, the crystallization is difficult because of the tendency of the material to oil.
10. This material is satisfactory for most synthetic purposes without further purification.
11.
Methylisourea hydrochloride decomposes on heating with evolution of
methyl chloride. The decomposition temperature depends on the rate of heating, but reproducible values are obtainable if the rate of heating is controlled. Samples of pure (
98–99%) (Note
12)
methylisourea hydrochloride, introduced into the
melting-point tube without undue previous exposure to atmospheric moisture, placed in the melting-point bath at 60°, and heated at the rate of 12° per minute, sinter at 118–119° and decompose at
122–124° (the mass moving rapidly up in the melting-point tube).
Owing to the presence of small quantities of free
hydrochloric acid in the crude product, the above procedures are applicable to recrystallized specimens only.
13. It has been suggested
2 that it is best to cool the solution to −10°, cool the wash solvent to −15° in ice-salt, and to put some of the cold solvent through the funnel before filtration (the addition of a few chips of Dry Ice to this solvent on the funnel is helpful).
3. Discussion
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