Urea or Carbamide

Urea gives a remarkable condensation product with formalde hyde known as "Pollopas," a transparent, colourless, lustrous solid resembling flint glass in appearance and refractive index, but dif fering from glass in that it is non-splintering and can be worked on a lathe. (See RESINS: Synthetic.) With hydrogen peroxide urea forms a crystalline additive com pound, employed as a disinfectant and as an oxidising agent. Urea finds employment as a stabiliser in nitrocellu lose explosives and has many medicinal uses such as an antipyretic and a diuretic. Urea salicylate (ursal) is recommended in gout and rheumatism. Urea acetylsalicylate and urea quinate (urol or uracol) have similar uses, and urea calcium briffnide, CaBr2,4C0(NH2)2, known as u;eabromin, is prescribed in epilepsy and neurasthenia.

Urea is the starting point in the manufacture of many important synthetic drugs. Veronal (q.v.) or diethylmalonyl urea, is the condensation product of urea and diethylmalonyl chloride ; its sodium salt is medicinal. Proponal is dipropylmalonyl urea and bromural is a-bromoisovaleryl urea.

Chemical Constitution of Urea.

Urea has long been re garded as the diamide of carbonic acid (carbamide) with formula I., but E. A. Werner has brought together considerable evidence in favour of the cyclic formula II., and he visualises also the transient existence of an active tautomeric enol form (formula III.) which has been stabilised in certain isourea derivatives.

When heated slightly above its melting point, urea dissociates into ammonia and the keto-form, NH:CO, of cyanic acid: This acid acting on unchanged urea produces biuret, At 195° C biuret decomposes into ammonia, cyanic acid, which polymerises to cyanuric acid, and ammelide, HN.CO.NHC(NH).NHCO. The transformation of ammonium cyanate to urea in boiling aqueous solution is a bimolecular revers ible reaction, and equilibrium is reached a decinormal solution with 95% of urea and 5% of cyanate. At 8o° C dry ammonium cyanate passes rapidly into urea, which is also produced by hy drolysis of lead cyanate (A. C. Cumming, 1903), and together with

biuret by heating under pressure a solution of carbon monoxide in ammoniacal cuprous chloride (Jouve, 1899).

Thiourea,

the sulphur analogue of urea, is now prepared on a manufacturing scale for employment in photography, synthetic resins, the treatment of weighted silk fabrics and as a starting material in the production of important azo-dyes and sulphur colours. It may be prepared (I) by heating ammonium thio cyanate at 16o-17o° C, when a reversible reaction takes place proceeding to equilibrium with 25% of thiourea; (2) by the action of hydrogen sulphide on cyanamide.

Thiourea crystallises from water or alcohol in colourless, six sided prisms or silky needles melting at 172-7° C. When con densed with formaldehyde, thiourea forms insoluble products which form the basis of important synthetic resins (q.v.). These resins are used in making moulding powders and moulded articles including vases, bowls and tea services, which admit of decorative treatment in mottled colourings.

Isomerism among thiourea derivatives is more noticeable than with those of urea. Derivatives corresponding with formula I. are the true thiocarbamides, their isomerides (termed isothioureas) are derived from the active thiol form (III.) which is regarded by Werner as existing in equilibrium with the cyclic modification (II.).

In most of its reactions thiourea behaves as a thiol; with diazome thane it yields methyl isothiourea, NH and with chloracetic acid it gives rise to a complex thiolacetic acid, NH Moreover, thiourea reacts in the thiol form with chloroacetaldehyde to furnish aminothioazole. (See THIAZOLE.) Both urea and thiourea are monacidic bases, a characteristic which is in accordance with Werner's view of their constitution. Urea nitrate is NH :C(OH)NH3NO3 and thiourea hydrochloride is NH