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urea (yo͝oēˈə), organic compound that is the principal end product of nitrogen metabolism in most mammals. Urea was the first animal metabolite to be isolated in crystalline form; its crystallization was described in the early 18th cent., and in 1773 it was noted that urea gave off ammonia when heated. This discovery provided a clue to its structure. In 1828 urea also became the first organic compound to be synthesized from inorganic materials (lead or silver cyanate and ammonia); this work was done by German chemist Friedrich Wöhler in 1828. Years of investigation of the biosynthesis of urea culminated in the proposal of the ornithine cycle (sometimes known as the Krebs urea cycle, named for German-born chemist Hans Krebs) in 1932. The proposed cycle has since been amended only in detail. It involves the linking of one molecule of ammonia with one molecule of carbon dioxide to form carbamoyl phosphate which then is added to ornithine resulting in the formation of citrulline. Next the nitrogen-containing amino group from aspartic acid is combined with the citrulline, resulting in the formation of arginine. The addition of a water molecule, arginine is then split into one molecule of urea and one molecule of ornithine, which can now repeat the cycle. In metabolism of proteins and other materials, the ammonia molecule that enters the cycle originates from glutamic acid, but glutamic acid can acquire the group that generates this ammonia from many other amino acids; thus most of the nitrogen in protein can eventually be converted to nitrogen in urea. These reactions have been shown to occur in the liver. Urea is transported in the blood to the kidneys, where it is filtered out; its concentration in urine is about 60 to 70 times as great as that in blood.
A colorless crystalline compound, formula CH4N2O, melting point 132.7°C (270.9°F). Urea is also known as carbamide and carbonyl diamide, and has numerous trade names as well. It is highly soluble in water and is odorless in its purest state, although most samples of even high purity have an ammonia odor. The diamide of carbonic acid, urea has the structure below.
Urea occurs in nature as the major nitrogen-containing end product of protein metabolism by mammals, which excrete urea in the urine. The adult human body discharges almost 50 g (1.8 oz) of urea daily. Urea was first isolated in 1773 by G. F. Rouelle. By preparing urea from potassium cyanate (KCNO) and ammonium sulfate (NH4SO4) in 1828, F. Wöhler achieved a milestone, the first synthesis of an organic molecule from inorganic starting materials, and thus heralded the modern science of organic chemistry. See Nitrogen, Protein metabolism
Because of its high nitrogen content (46.65% by weight), urea is a popular fertilizer. About three-fourths of the urea produced commercially is used for this purpose. After application to soil, usually as a solution in water, urea gradually undergoes hydrolysis to ammonia (or ammonium ion) and carbonate (or carbon dioxide). Another major use of urea is as an ingredient for the production of urea-formaldehyde resins, extremely effective adhesives used for laminating plywood and in manufacturing particle board, and the basis for such plastics as malamine. See Fertilizer, Urea-formaldehyde resins
Other uses of urea include its utilization in medicine as a diuretic. In the past, it was used to reduce intracranial and intraocular pressure, and as a topical antiseptic. It is still used for these purposes, to some extent, in veterinary medicine and animal husbandry, where it also finds application as a protein feed supplement for cattle and sheep. Urea has been used to brown baked goods such as pretzels. It is a stabilizer for nitrocellulose explosives because of its ability to neutralize the nitric acid that is formed from, and accelerates, the decomposition of the nitrocellulose. Urea was once used for flameproofing fabrics. Mixed with barium hydroxide, urea is applied to limestone monuments to slow erosion by acid rain and acidic pollutants.
(or carbamide), H2NCONH2, a diamide of carbonic acid, the amide of carbamic acid. Colorless crystals. Melting point, 132.7°C. Readily soluble in water, alcohol, liquid ammonia, and sulfur dioxide. Urea was discovered in urine by the French chemist H. Rouelle in 1773. It was identified by the English chemist W. Prout in 1818 and first synthesized by F. Wöhler in 1828 by heating ammonium cyanate, NH4NCO. This discovery was the first counterblow to the idealist vitalist doctrine of a “vital force.”
Urea is a highly reactive compound. It forms complexes with many substances—for example, with hydrogen peroxide, CO (NH2)2·H2O2, and with normal alkanes. The latter reaction is used in industry for deparaffination of petroleum. Upon heating to 150°-160°C, urea decomposes, with the formation of biuret (H2NCONHCONH2), NH3, carbon dioxide, and other products. Upon heating in aqueous solutions, urea slowly hydrolyzes to CO2 and NH3 (hydrolysis is rapid in the presence of acids or alkalies). With acids (HNO3 And Hc1), urea gives salts, such as CO (Nh2)2·HNO3. Alkyl ureas, Rnhconh2, are formed upon alkylation, and ureides, RCONHCONH2, are produced upon acylation. urea reacts with alcohols to yield urethanes, H2NCOOR. Urea condenses readily with formaldehyde. A hydrogen atom in the NH2 group may also be replaced by a halogen atom (F, Cl).
Urea is the end product of protein metabolism in most vertebrates and in man. It is found in the blood, muscles, saliva, lymph, milk, and other body fluids and tissues (the normal urea content in human blood is 18–38 mg/100 ml). The biosynthesis of urea from the end products of protein catabolism, NH3 and CO2, takes place in the liver as the result of a series of biochemical reactions called the urea cycle, or ornithine cycle (urea and ornithine are formed upon enzymatic cleavage of the amino acid arginine). In animals that bind ammonia in uric acid, the ornithine cycle is lost.
Urea participates in the regulation of the water balance of animals: it maintains the hypertonicity of tissues in sharks and provides tissue hydration in terrestrial animals. Urea is excreted by the kidneys and sweat glands (in man, about 25–30 g of urea is excreted per day). The content of urea in urine depends on the quantity and composition of proteins in the diet and on the level of protein catabolism (the content increases during physical labor and in cases of increased body temperature and diabetes mellitus). In cases of kidney dysfunction and diseases related to increased catabolism of tissue proteins, the content of urea in the blood increases (uremia). Urea and the ornithine cycle are found in fungi and the higher plants.
In industry, urea is obtained from ammonia and carbon dioxide at 160°-200°C and 100–400 technical atmospheres (96.7–386.8 atmospheres):
2NH3 + CO2 → [H2NCOONH4] → H2NNCONH2 + H2O
Urea has many uses. It is the raw material for the production of carbamide resins, as well as fertilizers, cyanates, hydrazine, cyanuric acid and its esters, a number of dyes, and soporifics (for example, Veronal, Luminal, and Bromural). In medical practice, pure urea is used as a diuretic for prevention and reduction of brain edema.
V. N. FROSIN and E. N. SAFONOVA
In agriculture, urea is one of the best concentrated nitrogen fertilizers. It contains 46 percent nitrogen and is weakly hygroscopic. Urea is produced mainly in granulated form; it does not cake, and it spreads well. Urea is used for presowing fertilization and as a top dressing (including foliar application) for many agricultural crops on all soils. The greatest increases in crop yield are obtained on moist soddy podzols and under irrigation conditions upon application to sugar beets, vegetable crops, and potatoes. In the first stomach of ruminants there is an abundance of microorganisms capable of using urea for the biosynthesis of protein, and therefore urea is added to fodder as a protein substitute.