To a solution of 16.8 g. (0.40 mole) of cyanamide in 50 ml. of water (Note 1) contained in a 400-ml. flask or beaker is added 50 ml. of aqueous 3N sodium hydroxide (0.15 mole). To the resulting solution, cooled to 15–18°, is added, in 2-ml. portions with shaking over a 15–18 minute period, 23.8 g. (22.0 ml., 0.2 mole) of phenyl isocyanate. The isocyanate dissolves rapidly, while the temperature rises slightly. The mixture is maintained at 20–25° by occasional external cooling in ice water. When half the phenyl isocyanate has been introduced, a second portion of 50 ml. (0.15 mole) of 3N sodium hydroxide is added to keep the reaction mixture strongly alkaline throughout the experiment. When addition is complete (Note 2), the slightly turbid liquid is diluted with 40 ml. of water and is filtered immediately under reduced pressure (Note 3) to remove undissolved impurities and traces of separated diphenylurea (Note 4). The cyanourea is precipitated from the clear colorless filtrate by the slow addition of concentrated hydrochloric acid with stirring until a permanent turbidity just appears (Note 3). Cracked ice (30–40 g.) is added to lower the temperature to 18–20°. Precipitation is then completed in this temperature range by the alternate addition of concentrated hydrochloric acid (total volume required, approximately 30 ml.) and cracked ice until the suspension is acid to Congo red. The crude cyanourea forms a microcrystalline, white precipitate, which, after storage at 0° for 3 hours, is collected by filtration under reduced pressure (Note 5) and is washed with two portions of cold water (20–25 ml.) (Note 6). The crude product is drained thoroughly, air-dried at room temperature, and finally dried to constant weight in a desiccator over phosphorus pentoxide. The yield of crude 1-cyano-3-phenylurea (Note 7), m.p. 122–126° with decomposition (Note 8), varies between 29 and 30.5 g. (90–95%). The dried material is purified as follows. A solution of the crude product in 100 ml. of boiling acetone is diluted slowly with gentle swirling with 30–40 ml. of petroleum ether (boiling range 40–60°). As crystallization proceeds, an additional 20–30 ml. of petroleum ether is added carefully at such a rate that the supernatant liquid does not become turbid. After 15 minutes at room temperature, the mixture is set aside at 0°. The product is collected by filtration under reduced pressure and washed successively with 50 ml. of an acetone-petroleum ether mixture (1:3) and 50 ml. of petroleum ether. 1-Cyano-3-phenylurea thus obtained forms colorless lustrous needles, m.p. 127–128° with decomposition (Note 8), yield 20–21.5 g. (62–67%). Slow dilution of the filtrates with petroleum ether to a total volume of 350–400 ml. and storage at 0° affords a second crop, m.p. 123–126° (dec.), of satisfactory purity; yield 4–5 g. (12–16%). The synthesis is generally applicable to the preparation of 1-cyano-3-arylureas (Note 9).