PREPARATIONS 125 filtrate on cooling deposits fine colourless crystals of monophenyl-urea. Filter off at the pump and then recrystallise from hot water: slow cooling of the hot aqueous solution causes the monophenylurea to separate in large crystals. Yield, 3*0-3-5 g.;m.p. 147°. Monophenyl-and Diphenyl-urea. (Second Method.) These substances, having the formula C 6 H 5 NHCONH, and OC(NHC.H 5 ) 2 respectively, are both formed when an aqueous solution of urea and aniline hydrochloride is heated. Their subsequent separation is based on the fact that diphenylurea is insoluble in boiling water, whereas monophenylurea is readily soluble. The formation of these compounds can be explained as follows. When urea is dissolved in water, a small proportion of it undergoes molecular rearrangement back to ammonium cyanate, an equilibrium thus being formed. NH,CONH t ^ NH 4 CNO The ammonium cyanate then reacts with the aniline hydrochloride giving aniline cyanate which, as in the previous preparation, gives in turn monophenyl-
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