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Org. Synth. 1923, 3, 95
DOI: 10.15227/orgsyn.003.0095
PHENYLUREA and sym.-DIPHENYLUREA
[Urea, phenyl-, and Carbanilide]
Submitted by Tenney L. Davis and K. C. Blanchard.
Checked by C. S. Marvel and W. B. King.
1. Procedure
A solution of 390 g. (3 moles) of aniline hydrochloride and 190 g. (3.2 moles) of urea in 1500 cc. of water is boiled in a 3-l. flask under a reflux condenser (Note 1) and (Note 2). After about one hour, crystals begin to separate; at the end of one and one-half to two hours, the mixture, which bumps considerably, is filtered rapidly by suction, and the crystals of carbanilide washed with 100 cc. of boiling water. The filtrate is chilled, and the phenylurea which crystallizes out is filtered off and rinsed with a little cold water, the washings being discarded. The filtrate is again boiled under a reflux condenser for one and one-half to two hours, or until it begins to bump, and the process is repeated. The above operations are repeated a third time, and each time the phenylurea is collected from the cold filtrate. The mother liquor is finally evaporated to one-half its original volume, and additional crops of carbanilide and phenylurea are so obtained. Further evaporation is not profitable.
The crude carbanilide (symmetrical diphenylurea) is practically pure, and may be obtained in large colorless needles melting at 235° by recrystallization from alcohol with the use of decolorizing carbon; 1 l. of alcohol is required for every 25 g. of product (Note 3). The weight of the crude carbanilide is 122–128 g. (38–40 per cent of the theoretical amount).
The crude phenylurea is somewhat colored and contains a little carbanilide. It may be purified by dissolving in a minimum quantity of boiling water, adding a little decolorizing carbon, and filtering. When the filtrate first begins to cool, a flocculent precipitate of carbanilide is deposited; this must be filtered off while the liquid is still hot. The filtrate, on cooling, yields colorless stout needles or flakes of phenylurea melting at 147° (Note 4). The total weight of pure product is 212–225 g. (52–55 per cent of the theoretical amount).
2. Notes
1. The reaction can undoubtedly be explained by the formation of an equilibrium mixture of urea and ammonium cyanate in boiling solution; the ammonium cyanate reacts at the moment of its formation with aniline hydrochloride, yielding phenylurea. Phenylurea also undergoes a secondary reaction, involving the intermediate formation of phenyl isocyanate, which reacts with aniline hydrochloride, forming carbanilide; so that in order to obtain the best yield it is necessary to interrupt the process from time to time and remove the phenylurea first formed.
That the reaction proceeds to completion under the above conditions is shown by the fact that although the mixture ultimately becomes faintly alkaline (on account of the formation of ammonia by hydrolysis of the excess of urea) at no time does the mixture appear to contain aniline.
2. In the above directions the concentration is as high as it is safe to make it; if less water is used the yield of phenylurea decreases and there is grave danger of loss due to the sudden heating of the mixture by rapid separation of crystals. This phenomenon has been observed with a more concentrated mixture even on heating on the steam bath.
3. The carbanilide can also be obtained in long needles from glacial acetic acid or preferably ethyl acetate.
4. If the solution of phenylurea is allowed to cool slowly, the product separates in needles several centimeters in length.
3. Discussion
Phenylurea can be prepared from aniline salts with potassium cyanate in aqueous solution,1 a method which suffers from the disadvantage that potassium cyanate is not easily prepared in the laboratory and rapidly undergoes spontaneous decomposition in storage; by heating aniline with urea,2 whereby carbanilide is formed in rather larger proportion than phenylurea; and from aniline and nitrourea in aqueous solution.3 A method analogous to the procedure described has been patented for the preparation of p-phenetylurea,4 in which p-phenetidine is heated with urea nitrate (or p-phenetidine hydrochloride with urea) in aqueous solution. This reaction appears to be generally applicable to aromatic primary amines; it does not, however, appear to be so well suited to the preparation of the corresponding derivatives of secondary amines.
This preparation is referenced from:

References and Notes
  1. Hofmann, Ann. 57, 265 (1846); Weith, Ber. 9, 820 (1876).
  2. Fleischer, Ber. 9, 995 (1876); Davis and Underwood, J. Am. Chem. Soc. 44, 2600 (1922).
  3. Davis and Blanchard, J. Am. Chem. Soc. 51, 1790 (1929).
  4. Riedel, Ger. pat. 76,596 [Frdl. 4, 1268 (1894–97)].

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

aniline salts

aromatic primary amines

secondary amines

sym.-DIPHENYLUREA

p-phenetylurea

alcohol (64-17-5)

acetic acid (64-19-7)

ammonia (7664-41-7)

ethyl acetate (141-78-6)

aniline (62-53-3)

aniline hydrochloride (142-04-1)

decolorizing carbon (7782-42-5)

urea (57-13-6)

Nitrourea (556-89-8)

urea nitrate

Phenylurea,
Urea, phenyl- (64-10-8)

diphenylurea (603-54-3)

Carbanilide (102-07-8)

ammonium cyanate

phenyl isocyanate (103-71-9)

potassium cyanate (590-28-3)

p-phenetidine (156-43-4)

p-phenetidine hydrochloride (637-56-9)