Urea

Urea
General
Systematic name	Diaminomethanal
Other names	carbamide
Molecular formula	(NH2)2CO
SMILES	NC(=O)N
Molar mass	60.07 g/mol
Appearance	white odourless solid
CAS number	[57-13-6]
Properties
Density and phase	1.33·103 kg/m3 , solid
Solubility in water	108 g/100 ml (20 °C) 167 g/100 ml (40 °C) 251 g/100 ml (60 °C) 400 g/100 ml (80 °C) 733 g/100 ml (100 °C)
Melting point	132.7 °C (406 K) decomposes
Boiling point	n.a.
Acidity (pKa)	0.18
Basicity (pKb)	13.82
Chiral rotation [α]D	Not chiral
Viscosity	? cP at ? °C
Critical relative humidity	81% (20°C) 73% (30°C)
Heat of solution in water	-57,8 cal/g (endothermic)
Nitrogen content	46,6 %N
Structure
Molecular shape	?
Coordination geometry	trigonal planar
Crystal structure	tetragonal
Dipole moment	4.56 p/D
Hazards
MSDS	J.T. Baker
Main hazards	Toxic
Flash point	? °C
R/S statement	R: ? S: ?
RTECS number	?
NFPA 704	0 1 0   estimated
Supplementary data page
Structure & properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	?
Other cations	?
Related ?	biuret triuret thiourea
Related compounds	?
Except where noted otherwise, data are given for materials in their standard state (at 26°C, 100 kPa) Infobox disclaimer and references

Urea is an organic compound of carbon, nitrogen, oxygen and hydrogen, with the formula CON₂H₄ or (NH₂)₂CO or CN₂H₄O.

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.

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 Wöhler, who prepared it by the reaction of potassium cyanate with ammonium sulfate. Although Wöhler 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.

Physiology

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.

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 different 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.

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.

Urea is essentially a waste product, although it is used by the body during times of volume reduction. In the later portions of the kidney collecting tubule, urea is reintroduced into the kidney medulla to raise osmolarity. Afterwards, water flowing through the collecting tubule follows back into the body by osmosis.

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.

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 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.

Commercial 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 is exothermic:

2NH₃ + CO₂ → H₂N-COONH₄ (ammonium carbamate)

While the second reaction is endothermic:

H₂N-COONH₄ → (NH₂)₂CO + H₂O

Both reactions combined are exothermic.

Uses

Commercial uses

• 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.
• As a NOx-reducing reactant in combustion exhaust streams, especially diesel.
• As an ingredient in many tooth whitening products.
• Used in coal fired power plants to reduce NO emissions.

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 uses

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).

Textile use

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 dyeing or printing process.

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.

See also

• Amide
• Ammonia
• Fertilizer
• Nitrogen

References

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External links

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

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