UREA. 757 UREAS. acid produced in the decomposition, and hence, by a simple calculationj the weight of uioa decom- posed. The presence of urea may be readily de- tected by means of the biuret reaction. It has been mentioned above that wlicn urea is decom- posed by heating, biuret is produced along with several other products of decomposition. Now, when biuret is dissolved in water, then some caus- tic soda and a little dilute copper sulphate so- lution added, a characteristic pink coloration is produced. To detect urea, therefore, the dry substance submitted for examination, or the residue left on evaporating a solution, is heated for some time at 150° C, then dissolved and treated with caustic soda and copper sulphate, a pink coloration produced proving the presence of urea. Urea was discovered in urine in 1773, and it was the first organic compound produced arti- ficially in the chemical laboratory — a synthesis of vast importance in the history of chemistry. (See Chemistry.) It was then (1S2,S) obtained by Wohler (q.v. ) by heating a solution of am- monium cyanate, the transformation taking place according to the following chemical equation: NH^OCN" = C0{NHj)2 Ammonium Urea, c.vanate Urea has since been made artificially in a variety of ways; for instance, by treating am- monia with phosgene gas (carbonyl chloride), the following reaction readily taking place: COCI2 + 4NH, = CO(NH2)2 + 2NH4C1 Phosgene .mmonia Urea Ammonium gas chloride. This synthesis is important inasmuch as it ex- plains the structure of urea, the molecule of the latter evidently consisting of one carbonyl (CO) group and two NH, groups; therefore, urea is considered as the diamide of carbonic acid [CO(OH),] and is chemically called carbamide. Urea is best prepared in the laboratory by heating phenyl carbonate with ammonia : (CHJ^CO^V 2NH3 = CO(NH,), + 2C,H,0H. It may be obtained from urine by evaporating to a small volume and adding nitric acid ; the nitrate of urea thus obtained is purified by crystalliza- tion from nitric acid, decomposed with barium carbonate, and the urea set free is dissolved out of the mixture thus obtained with alcohol. Urea occurs more abundantly in the urine of carnivorous than in that of herbivorous animals. Small quantities of urea are present also in the urine of birds. The average amount of urea normally excreted by the human adult in a day is 33 grams, the quantity varying between 22 and 35 grams. Urea is also a constituent of the fluids of the eye, of the sweat, and. in minute quantity, of the blood and of the licpior amnii (of the fo?tus). There can be no doubt that it is a final product of the regressive metamorjihosis of the living tissues, or of their disintegration into simpler compounds, by means of which the final elimination of the worn-out structures is eiTeeted. In what way this process of disinte- gration takes place in the animal body is not known; though various hypotheses have been ad- vanced in explanation of this important phenom- enon. Since urea is readily produced from am- monium cyanate and is with equal readiness transformed into ammoniiun carbonate (see above), it may be that the decomposition of pro- teids in the body gives rise first to the forma- tion of one or both of the.se substances, which are subsequently changed into urea. Such ex- planations, however, are rather vague; for, al- though cyanogen compounds are manufactured for practical purposes chielly from nitrogenous animal refu.se, the nature of the chemical trans- formations taking place during the process is not luulerstood. The cjanate hypotlicsis is atti'active inasmuch as the transformaticm of cyanogen compounds into urea might serve as a source of much energv', cyanogen compounds possessing great molecular energj', wliile urea possesses much less energy and is much more stable. UREAS, The Compo^xd. A large class of organic substances, for the knowledge of which chemistry is mainly indebted to A. W'. Ilofmann. It is stated in the article on Ukea (q.v.) that that important substance can be obtained by the action of ammonia either on cyanic acid (syn- thesis of urea by heating an ammonium cyanate solution), or on phosgene gas (carbonyl chloride). Similar transformations take place if. in place of ammonia, compounds are employed which are derived from ammonia by replacing hydrogen in the latter by alcohol radicles, such as methyl (CH3) or ethyl (CJI,,). In these cases, how- ever, not urea itself, but a series of compound ureas are obtained, which are said to be derived from urea by substituting an alcohol radicle for hydrogen. These compound ureas are very sim- ilar in their chemical behavior to urea itself, and, like urea, form salts with acids. Most of the ureas are crystalline solid substances. Those, however, which are derived by replacing all the hydrogen of urea are liquids that boil at high temperatures and distil without decomposition. Ethyl urea and Ictra-mctht/l urea are formed by the above methods, according to the following chemical equations : NHC2H5 / HNCO + NH.CHs = CO
NH, Cyanic acid Ethylamine Etlivl urea. (NCH,), / COClo + 2NH(CH3), = CO + 2HC1
N(CH3), Carbonyl Di-methyl- Tetra-niethyl chloride amine lirea. Certain compound ureas may be obtained by using ethereal salts of cyanic acid; thus sym- metrical methyl-ethyl urea is obtained by the action of methylamine on ethyl cyanate. Ihc re- action taking place according to the following chemical equation: NHC2H5 CoH^NCO-f NHjCHj = CO
NHCH, Methyl- amine Methyl-ethyl urea. Ethyl cyanate Another important series of compound ureas is obtained by substituting for hydrogen in urea the radicles of organic acids. These ureas are called ureidcs. One of the hydrogen atoms of urea can be replaced by an acid radicle by the
