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Urea is an organic compound of carbon, nitrogen, oxygen and hydrogen, with the formula (NH<sub>2</sub>)<sub>2</sub>CO.
Urea is also known as carbamide, especially in the recommended International Nonproprietary Names (rINN) in use in Europe. For example, the medicinal compound hydroxyurea (old British Approved Name) is now hydroxycarbamide. Other names include carbamide resin, isourea, carbonyl diamide, and carbonyldiamine.
It was the first organic compound to be artificially synthesized from inorganic starting materials, thus dispelling the concept of vitalism.
Urea was discovered by Hilaire Rouelle in 1773. It was the first organic compound to be artificially synthesized from inorganic starting materials, in 1828 by Friedrich Woehler, who prepared it by the reaction of potassium cyanate with ammonium sulfate. Although Woehler was attempting to prepare ammonium cyanate, by forming urea, he inadvertently disproved vitalism, the theory that the chemicals of living organisms are fundamentally different from inanimate matter, thus starting the discipline of organic chemistry.
It is found in mammalian and amphibian urine as well as in some fishes. Birds and reptiles excrete uric acid, comprising a different form of nitrogen metabolism that requires less water.
The individual atoms that make up a urea molecule come from carbon dioxide, water, aspartate and ammonia in a metabolic pathway known as the urea cycle, an anabolic process. This expenditure of energy is necessary because ammonia, a common metabolic waste product, is toxic and must be neutralized. Urea production occurs in the liver and is under the regulatory control of N-acetylglutamate.
The urea cycle was originally known as the Krebs-Henseleit cycle after it was partially deduced by Hans Adolf Krebs and Kurt Henseleit in 1932. Its details were clarified in the 1940s as the roles of citrulline and argininosuccinate as intermediates were understood. In this cycle, amino groups donated by ammonia and L-aspartate are converted to urea, while L-ornithine, citrulline, L-argininosuccinate, and L-arginine act as intermediates.
Most organisms have to deal with the excretion of nitrogen waste originating from protein and amino acid catabolism. In aquatic organisms the most common form of nitrogen waste is ammonia, while land-dwelling organisms developed ways to convert the toxic ammonia to either urea or uric acid. Generally, birds and saurian reptiles excrete uric acid, while the remaining species, including mammals, excrete urea. Remarkably, tadpoles excrete ammonia, and shift to urea production during metamorphosis. In veterinary medicine, Dalmatian breeds of dogs are noteworthy in that they excrete urea in the form of uric acid in the urine rather than in the urea form. This is due to a defect in one of the genes controlling expression of the conversion enzymes in the urea cycle.
Despite the generalization above, the pathway has been documented not only in mammals and amphibians, but in many other organisms as well, including birds, invertebrates, insects, plants, yeast, fungi, and even microorganisms.
Urea is essentially a waste product, but is vital for forming hypertonic (concentrated) urine. In the distal portions of the kidney collecting duct, urea is reintroduced into the kidney medulla to raise osmolarity. Afterwards, water flowing through the collecting tubule follows back into the body by osmosis through aquaporins.
Urea is dissolved in blood (in humans in a concentration of 2.5 - 7.5 mmol/liter) and excreted by the kidney in the urine.
In addition, a small amount of urea is excreted (along with sodium chloride and water) in human sweat.
Urea is a nitrogen-containing chemical product which is produced on a scale of some 100,000,000 tonnes per year worldwide.
Urea is produced commercially from synthetic ammonia and carbon dioxide. Urea can be produced as prills, granules, flakes, pellets, crystals and solutions.
More than 90% of world production is destined for use as a fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use (46.4%) It therefore has the lowest transportation costs per unit of nitrogen nutrient.
Urea is highly soluble in water and is therefore also very suitable for use in fertilizer solutions (in combination with ammonium nitrate: UAN), e.g. in 'foliar feed' fertilizers.
Solid urea is marketed as prills or granules. The advantage of prills is that in general they can be produced more cheaply than granules which, because of their narrower particle size distribution have an advantage over prills if applied mechanically to the soil. Properties such as impact strength, crushing strength and free-flowing behaviour are particularly important in product handling, storage and bulk transportation.
Urea is produced commercially from two raw materials, ammonia and carbon dioxide. Large quantities of carbon dioxide are produced during the manufacture of ammonia from coal or from hydrocarbons such as natural gas and petroleum derived raw materials. This allows direct synthesis of urea from these raw materials.
The production of urea from ammonia and carbon dioxide takes place in an equilibrium reaction, with incomplete conversion of the reactants. The various urea processes are characterized by the conditions under which urea formation takes place and the way in which unconverted reactants are further processed.
Unconverted reactants can be used for the manufacture of other products, for example ammonium nitrate or sulphate, or they can be recycled for complete conversion to urea in a total-recycle process.
Two principal reactions take place in the formation of urea from ammonia and carbon dioxide. The first reaction is exothermic: 2NH<sub>3</sub> + CO<sub>2</sub> → H<sub>2</sub>N-COONH<sub>4</sub> (ammonium carbamate) <br /> While the second reaction is endothermic: H<sub>2</sub>N-COONH<sub>4</sub> → (NH<sub>2</sub>)<sub>2</sub>CO + H<sub>2</sub>O Both reactions combined are exothermic.
The process is also called the Bosch-Meiser urea process after its discoverers (1922).
Urea's commercial uses include:
Urea is a powerful protein denaturant. This property can be exploited to increase the solubility of some proteins. For this application it is used in concentrations up to 10 M. Urea is used to effectively disrupt the noncovalent bonds in proteins. Urea is an ingredient in the synthesis of urea nitrate.
Urea is used in topical dermatological products to promote rehydration of the skin. If covered by an occlusive dressing, 40% urea preparations may also be used for nonsurgical debridement of nails.
See blood urea nitrogen ("BUN") for a commonly performed urea test, and marker of renal function.
Isotopically-labeled urea (carbon 14 - radioactive, or carbon 13 - stable isotope) is used in the Urea breath test, which is used to detect the presence of Helicobacter pylori (H. pylori, a bacterium) in the stomach and duodenum of humans. The test detects the characteristic enzyme urease, produced by H. pylori, by a reaction that produces ammonia from urea. This increases the pH (reduces acidity) of the stomach environment around the bacteria.
Similar bacteria species to H. pylori can be identified by the same test in animals (apes, dogs, cats - including big cats).
Urea in textile laboratories are frequently used both in dyeing and printing as an important auxiliary which provides solubility to the bath and retains some moisture which is required for the dying or printing process.
Ureas or carbamides are a class of chemical compounds sharing the same functional group RR'N-CO-NRR' based on a carbonyl group flanked by two organic amine residues. They can be accessed in the laboratory by reaction of phosgene with primary or secondary amines. Example of ureas are the compounds carbamide peroxide, allantoin and Hydantoin. Ureas are closely related to biurets and structurally related to amides, carbamates, diimides, carbodiimides and thiocarbamides.