urea

Urea is also known as carbamide, especially in the recommended International Non-proprietary 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.

Discovery

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.

Physiology

The individual atoms of urea 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.

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.

The urea is formed in the livers of mammals in a cyclic pathway, from the break down of ammonia, (a metabolic waste), which was initially named the Krebs-Henseleit cycle after its discoverers, and later became known simply as the urea cycle. This cycle was partially deduced by Krebs & Henseleit in 1932 and was 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-arginino-succinate, and L-arginine act as intermediates.

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.

Humans produce a little uric acid as a result of purine breakdown. Excess uric acid production can lead to a type of arthritis known as gout.

Urea is essentially a waste product; it has no physiological function. It is dissolved in blood (in humans in a concentration of 2.5 - 7.5 mmol/liter) and excreted by the kidney in the urine.

Many researchers used to believe that a small amount of urea was excreted (along with sodium chloride and water) in human sweat. However that was proved wrong when Dr. M. Falk determined at RC Institute that only sodium chloride and water are excreted from sweat.

Commercial production

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 nitrogeneous fertilizers in common use. (46.4%N.) 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.

Production

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 (2NH₃ + CO₂ --> H₂N-CO-NH₃OH (ammonium carbamate)) is exothermic and the second reaction (H₂N-CO-NH₃OH (ammonium carbamate) --> H₂N-CO-NH₂ + H₂O) is endothermic. Both reactions combined are exothermic.

Industrial use

Urea's commercial uses include:

• As a raw material for the manufacture of plastics specifically, urea-formaldehyde resin.
• As a raw material for the manufacture of various glues (urea-formaldehyde or urea-melamine-formaldehyde). The latter is waterproof and is used for marine plywood.
• As a component of fertilizer and animal feed, providing a relatively cheap source of fixed nitrogen to promote growth.
• As an alternative to rock salt in the deicing of roadways and runways. It does not promote metal corrosion to the extent that salt does.
• As an additive ingredient in cigarettes, designed to enhance flavour.
• Sometimes used as a browning agent in factory-produced pretzels.
• As an ingredient in some hair conditioners, facial cleansers, bath oils and lotions.
• It is also used as a reactant in some ready-to-use cold compresses for first-aid use, due to the endothermic reaction it creates when mixed with water.
• Active ingredient for diesel engine exhaust treatment AdBlue and some other SCR systems.
• Used, along with salts, as a cloud seeding agent to expedite the condensation of water in clouds, producing precipitation.
• The ability of urea to form clathrates (also called host-guest complexes, inclusion compounds, and adducts) was used in the past to separate paraffins.
• As a flame-proofing agent.
• As a clean burning fuel for motor vehicles and stationary engines.

Laboratory use

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.

Medical use

Drug use

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.

Clinical diagnosis

See blood urea nitrogen ("BUN") for a commonly performed urea test, and marker of renal function.

Other diagnostic use

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).

Ureas

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.

External links

• MSDS sheet on urea
• Use of urea in hand dyeing
• Urea: synonymes, CAS, formula
• U3K Energy: Patented technolgy for use of urea as fuel (not SCR additive) for internal combustion engines and fuel cells.

• Link page to external chemical sources.