Carbamide, CO(NH₂)₂

Carbamide, or Urea, CO(NH₂)₂, the amide of carbonic acid, may be briefly noticed here.

Urea was discovered by Rouelle in 1773, investigated by Fourcroy and Vauquelin in 1790, and prepared from ammonium cyanate by Wohler in 1828:

NCONH₄ = CO(NH₂)₂.

Urea may be prepared from urine by evaporation and extraction of the residue with alcohol; or by precipitating from the concentrated urine the sparingly soluble nitrate or oxalate, which are subsequently decomposed by potassium carbonate and chalk respectively.

Like other amides, carbamide can be prepared by the action of ammonia on the corresponding acid chloride, which is carbonyl chloride or phosgene, COCl₂:

COCl₂ + 4NH₃ = CO(NH₂)₂ + 2NH₄Cl;

urea may also be prepared from ammonium carbamate, which loses water when heated to 130° to 140° C.:

H₄NO-CO-NH₂ = H₂N-CO-NH₂ + H₂O

but it is best obtained from ammonium cyanate according to the transformation discovered by Wohler. For this purpose potassium cyanate is first prepared by oxidising potassium cyanide by fusing it with red lead, or by the oxidation of potassium ferrocyanide by means of potassium dichromate. The potassium cyanate is then extracted with water and evaporated with its equivalent of ammonium sulphate; and from the mixture of potassium sulphate and urea which forms the residue the latter is extracted by means of alcohol, from which it may be crystallised.

Urea crystallises in quadratic prisms which melt at 132° C. and decompose at a higher temperature. The transformation of ammonium cyanate into urea is a reversible reaction, and the mechanism of the change has been the subject of much research and theorising.

Urea is very soluble in hot and cold water; it dissolves in 5 parts of cold and in 1 part of boiling alcohol, but is practically insoluble in ether.

Being the amide of a weak acid, urea has distinctly basic properties, although its aqueous solution is neutral in reaction.

Urea hydrochloride, CO(NH₂)₂.HCl, is formed with the evolution of heat by the union of HCl gas with urea in absence of water. It forms crystals which are completely decomposed by water.

Urea nitrate, CO(NH₂)₂.HNO₃, though moderately soluble in water, is almost insoluble in nitric acid, and is therefore formed as a crystalline precipitate when this acid is added to a sufficiently concentrated solution of urea; the formation of this salt is a characteristic reaction of urea.

Urea oxalate, 2CO(NH₂)₂.H₂C₂O₄, crystallises in monoclinic plates when concentrated solutions of urea and oxalic acid are mixed.

When urea is heated above its melting-point it loses ammonia and is converted into biuret, NH₂-CO-NH-CO-NH₂, and other products. When biuret is dissolved in alkali and a drop of dilute copper sulphate solution is added to the liquid, a purple colour is produced. This reaction constitutes a valuable test for urea. Like other amides, urea is not decomposed by cold alkali, but when warmed with alkali hydroxide solution it forms alkali carbonate with evolution of ammonia. Urea behaves similarly to ammonium salts towards hypochlorite or hypobromite and nitrite solutions, i.e. its nitrogen is evolved in the gaseous state. Thus with hypobromite solution the following reaction takes place:

CO(NH₂)₂ + 3NaOBr = 3NaBr + 2H₂O + CO₂ + N₂. The measurement of the nitrogen evolved in this reaction serves to estimate urea, though experience shows that the volume of nitrogen liberated corresponds to only 92 per cent, of the urea.

The similarity between the reactions of ammonia and urea towards nitrous acid is shown by the equations:

2NH₃ + N₂O₃ = 2N₂ + 3H₂O
CO(NH₂)₂ + N₂O₃ = 2N₂ + 2H₂O + CO₂.

The second equation represents the reaction between warm solutions of urea and nitrite; if the solutions are cold only half the amino- nitrogen reacts with the nitrite and the remainder appears as ammonium carbonate, thus:

2CO(NH₂)₂ + N₂O₃ = 2N₂ + (NH₄)₂CO₃ + CO₂.

Concentrated hydrochloric acid converts urea into carbon dioxide and ammonium chloride, cyanic acid being an intermediate product.