Urea

Urea, also called carbamide, is an organic compound with chemical formula. This amide has two amino groups joined by a carbonyl functional group. It is thus the simplest amide of carbamic acid.
Urea serves an important role in the cellular metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. Urea is Neo-Latin,,, itself from Proto-Indo-European *h₂worsom.
It is a colorless, odorless solid, highly soluble in water, and practically non-toxic. Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two ammonia molecules with a carbon dioxide molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen and is an important raw material for the chemical industry.
In 1828, Friedrich Wöhler discovered that urea can be produced from inorganic starting materials, an important conceptual milestone in chemistry. This showed for the first time that a substance previously known only as a byproduct of life could be synthesized in the laboratory from non-biological starting materials, thereby contradicting the widely held doctrine of vitalism, which stated that organic compounds could only be derived from living organisms.

Properties

Molecular and crystal structure

The structure of the molecule of urea is. The urea molecule is planar when in a solid crystal because of sp² hybridization of the N orbitals. It is non-planar with C₂ symmetry when in the gas phase or in aqueous solution, with C–N–H and H–N–H bond angles that are intermediate between the trigonal planar angle of 120° and the tetrahedral angle of 109.5°. In solid urea, the oxygen center is engaged in two N–H–O hydrogen bonds. The resulting hydrogen-bond network is probably established at the cost of efficient molecular packing: The structure is quite open, the ribbons forming tunnels with square cross-section. The carbon in urea is described as sp² hybridized, the C-N bonds have significant double bond character, and the carbonyl oxygen is relatively basic. Urea's high aqueous solubility reflects its ability to engage in extensive hydrogen bonding with water.
By virtue of its tendency to form porous frameworks, urea has the ability to trap many organic compounds. In these so-called clathrates, the organic "guest" molecules are held in channels formed by interpenetrating helices composed of hydrogen-bonded urea molecules. In this way, urea-clathrates have been well investigated for separations.

Reactions

Urea is a weak base, with a pK_b of 13.9. When combined with strong acids, it undergoes protonation at oxygen to form uronium salts. It is a Lewis base, forming metal complexes of the type.
Urea reacts with malonic esters to make barbituric acids.

Thermolysis

Molten urea decomposes into ammonium cyanate at about 152 °C, and into ammonia and isocyanic acid above 160 °C:
Heating above 160 °C yields biuret and triuret via reaction with isocyanic acid:
At higher temperatures it converts to a range of condensation products, including cyanuric acid, guanidine, and melamine.

Aqueous stability

In aqueous solution, urea slowly equilibrates with ammonium cyanate. This elimination reaction cogenerates isocyanic acid, which can carbamylate proteins, in particular the N-terminal amino group, the side chain amino of lysine, and to a lesser extent the side chains of arginine and cysteine. Each carbamylation event adds 43 daltons to the mass of the protein, which can be observed in protein mass spectrometry. For this reason, pure urea solutions should be freshly prepared and used, as aged solutions may develop a significant concentration of cyanate. Dissolving urea in ultrapure water followed by removing ions with a mixed-bed ion-exchange resin and storing that solution at 4 °C is a recommended preparation procedure. However, cyanate will build back up to significant levels within a few days. Alternatively, adding 25–50 mM ammonium chloride to a concentrated urea solution decreases formation of cyanate because of the common ion effect.

Analysis

Urea is readily quantified by a number of different methods, such as the diacetyl monoxime colorimetric method, and the Berthelot reaction. These methods are amenable to high throughput instrumentation, such as automated flow injection analyzers and 96-well micro-plate spectrophotometers.

Related compounds

Urea is the parent for a class of chemical compounds that share the same functional group. Namely, such compounds have a carbonyl group attached to two organic amine residues:, where groups are hydrogen, organyl or other groups. Examples include carbamide peroxide, allantoin, and hydantoin. Ureas are closely related to biurets and related in structure to amides, carbamates, carbodiimides, and thiocarbamides.

Uses

Agriculture

More than 90% of world industrial production of urea is for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has a low transportation cost per unit of nitrogen nutrient. Urea breaks down in the soil to give ammonium ions. The ammonium is taken up by the plant through its roots. In some soils, the ammonium is oxidized by bacteria to give nitrate, which is also a nitrogen-rich plant nutrient. The loss of nitrogenous compounds to the atmosphere and runoff is wasteful and environmentally damaging so urea is sometimes modified to enhance the efficiency of its agricultural use. Techniques to make controlled-release fertilizers that slow the release of nitrogen include the encapsulation of urea in an inert sealant, and conversion of urea into derivatives such as urea-formaldehyde compounds, which degrade into ammonia at a pace matching plants' nutritional requirements. The most common impurity of synthetic urea is biuret, which impairs plant growth.

Resins

Urea is a raw material for the manufacture of formaldehyde based resins, such as UF, MUF, and MUPF, used mainly in wood-based panels, for instance, particleboard, fiberboard, OSB, and plywood.

Explosives

Urea can be used in a reaction with nitric acid to make urea nitrate, a high explosive that is used industrially and as part of some improvised explosive devices.

Automobile systems

Urea is used in Selective Non-Catalytic Reduction and Selective Catalytic Reduction reactions to reduce the nitrogen oxide| pollutants in exhaust gases from combustion from diesel, dual fuel, and lean-burn natural gas engines. The BlueTec system, for example, injects a water-based urea solution into the exhaust system. Ammonia produced by the hydrolysis of urea reacts with nitrogen oxides and is converted into nitrogen gas and water within the catalytic converter. The conversion of noxious to innocuous is described by the following simplified global equation:
When urea is used, a pre-reaction occurs to first convert it to ammonia:
Being a solid highly soluble in water, urea is much easier and safer to handle and store than the more irritant, caustic and hazardous ammonia, so it is the reactant of choice. Trucks and cars using these catalytic converters need to carry a supply of diesel exhaust fluid, also sold as AdBlue, a solution of urea in water.

Laboratory uses

Urea in concentrations up to 10 M is a powerful protein denaturant as it disrupts the noncovalent bonds in the proteins. This property can be exploited to increase the solubility of some proteins. A mixture of urea and choline chloride is used as a deep eutectic solvent, a substance similar to ionic liquid. When used in a deep eutectic solvent, urea gradually denatures the proteins that are solubilized.
Urea in concentrations up to 8 M can be used to make fixed brain tissue transparent to visible light while still preserving fluorescent signals from labeled cells. This allows for much deeper imaging of neuronal processes than previously obtainable using conventional one photon or two photon confocal microscopes.

Medical use

s are used as topical dermatological products to promote rehydration of the skin. Urea 40% is indicated for psoriasis, xerosis, onychomycosis, ichthyosis, eczema, keratosis, keratoderma, corns, and calluses. If covered by an occlusive dressing, 40% urea preparations may also be used for nonsurgical debridement of nails. Urea 40% "dissolves the intercellular matrix" of the nail plate. Only diseased or dystrophic nails are removed, as there is no effect on healthy portions of the nail. This drug is also used as an earwax removal aid.
Urea has been studied as a diuretic. It was first used by Dr. W. Friedrich in 1892. In a 2010 study of ICU patients, urea was used to treat euvolemic hyponatremia and was found safe, inexpensive, and simple.
Like saline, urea has been injected into the uterus to induce abortion, although this method is no longer in widespread use.
The blood urea nitrogen test is a measure of the amount of nitrogen in the blood that comes from urea. It is used as a marker of renal function, though it is inferior to other markers such as creatinine because blood urea levels are influenced by other factors such as diet, dehydration, and liver function.
Urea has also been studied as an excipient in drug-coated balloon coating formulations to enhance local drug delivery to stenotic blood vessels. Urea, when used as an excipient in small doses to coat DCB surface was found to form crystals that increase drug transfer without adverse toxic effects on vascular endothelial cells.
Urea labeled with carbon-14 or carbon-13 is used in the urea breath test, which is used to detect the presence of the bacterium Helicobacter pylori in the stomach and duodenum of humans, associated with peptic ulcers. The test detects the characteristic enzyme urease, produced by H. pylori, by a reaction that produces ammonia from urea. This increases the pH of the stomach environment around the bacteria. Similar bacteria species to H. pylori can be identified by the same test in animals such as apes, dogs, and cats.