It is a colorless, odorless solid, highly soluble in water, and practically non-toxic ( is 15 g/kg for rats).
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.
The structure of the molecule of urea is . The urea molecule is planar when in a solid crystal because of sp
hybridization of the N orbitals.
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
. 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 sp
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 , the organic "guest" molecules are held in channels formed by interpenetrating helices composed of hydrogen bond urea molecules. In this way, urea-clathrates have been well investigated for separations.
of 13.9.
[ When combined with strong acids, it undergoes protonation at oxygen to form uronium salts.]
Urea reacts with malonic acid esters to make .
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
Analysis
Urea is readily quantified by a number of different methods, such as the diacetyl monoxime colorimetric method, and the Berthelot reaction (after initial conversion of urea to ammonia via urease). These methods are amenable to high throughput instrumentation, such as automated flow injection analyzers
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 (–H), organyl or other groups. Examples include carbamide peroxide, allantoin, and hydantoin. Ureas are closely related to and related in structure to , , , and .
Uses
Agriculture
More than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer.
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 (SNCR) and Selective Catalytic Reduction (SCR) reactions to reduce the nitrogen oxide in from combustion from Diesel fuel, 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:[Duo et al., (1992). Can. J. Chem. Eng, 70, 1014–1020.]
When urea is used, a pre-reaction (hydrolysis) occurs to first convert it to ammonia:
Being a solid highly solubility in water (545 g/L at 25 °C),
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 (DES), 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
Urea-containing creams are used as topical dermatology 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"
Urea has also 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 Euvolemia 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 (BUN) 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,
Urea has also been studied as an excipient in drug-coated balloon (DCB) coating formulations to enhance local drug delivery to stenotic blood vessels.
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 ( H. pylori) in the stomach and duodenum of humans, associated with . The test detects the characteristic enzyme urease, produced by H. pylori, by a reaction that produces ammonia from urea. This increases the pH (reduces the acidity) of the stomach environment around the bacteria. Similar bacteria species to H. pylori can be identified by the same test in animals such as , , and (including ).
Miscellaneous uses
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An ingredient in diesel exhaust fluid (DEF), which is 32.5% urea and 67.5% de-ionized water. DEF is sprayed into the exhaust stream of diesel vehicles to break down dangerous emissions into harmless nitrogen and water.
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A component of compound feed, providing a relatively cheap source of nitrogen to promote growth
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A non-corroding alternative to rock salt for road deicing.
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A main ingredient in hair removers such as Nair and Veet
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A browning agent in factory-produced
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An ingredient in some skin cream,
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A cloud seeding agent, along with other salts
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A flame-proofing agent, commonly used in dry chemical fire extinguisher charges such as the urea-potassium bicarbonate mixture
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An ingredient in many tooth whitening products
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An ingredient in dish soap
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Along with diammonium phosphate, as a yeast nutrient, for fermentation of into ethanol
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A nutrient used by plankton in ocean nourishment experiments for geoengineering purposes
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As an additive to extend the working temperature and open time of hide glue
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As a solubility-enhancing and moisture-retaining additive to dye baths for textile dyeing or printing
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As an optical parametric oscillator in nonlinear optics
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To help prepare a alpine skiing course by hardening the snow into a icier surface to maintain the integrity of the course.
Physiology
Amino acids from ingested food (or produced from catabolism of muscle protein) that are used for the synthesis of proteins and other biological substances can be oxidized by the body as an alternative source of energy, yielding urea and carbon dioxide.
Ammonia () is a common byproduct of the metabolism of nitrogenous compounds. Ammonia is smaller, more volatile, and more mobile than urea. If allowed to accumulate, ammonia would raise the pH in cells to toxic levels. Therefore, many organisms convert ammonia to urea, even though this synthesis has a net energy cost. Being practically neutral and highly soluble in water, urea is a safe vehicle for the body to transport and excrete excess nitrogen.
Urea is synthesized in the body of many organisms as part of the urea cycle, either from the oxidation of or from ammonia. In this cycle, amino groups donated by ammonia and -aspartate are converted to urea, while -ornithine, citrulline, -argininosuccinate, and -arginine act as intermediates. Urea production occurs in the liver and is regulated by N-acetylglutamate. Urea is then dissolved into the blood (in the reference range of 2.5 to 6.7 mmol/L) and further transported and excreted by the kidney as a component of urine. In addition, a small amount of urea is excreted (along with sodium chloride and water) in sweat.
In water, the amine groups undergo slow displacement by water molecules, producing ammonia, , and . For this reason, old, stale urine has a stronger odor than fresh urine.
Humans
The cycling of and excretion of urea by the kidneys is a vital part of mammalian metabolism. Besides its role as carrier of waste nitrogen, urea also plays a role in the countercurrent exchange system of the , that allows for reabsorption of water and critical ions from the excreted urine. Urea is reabsorbed in the inner medullary collecting ducts of the nephrons,[ Page 837] thus raising the osmolarity in the medullary interstitium surrounding the thin descending limb of the loop of Henle, which makes the water reabsorb.
By action of the urea transporter 2, some of this reabsorbed urea eventually flows back into the thin descending limb of the tubule,
The equivalent nitrogen content (in ) of urea (in mmol) can be estimated by the conversion factor 0.028 g/mmol.[Section 1.9.2 (page 76) in: ] Furthermore, 1 gram of nitrogen is roughly equivalent to 6.25 grams of protein, and 1 gram of protein is roughly equivalent to 5 grams of muscle tissue. In situations such as muscle wasting, 1 mmol of excessive urea in the urine (as measured by urine volume in litres multiplied by urea concentration in mmol/L) roughly corresponds to a muscle loss of 0.67 gram.
Other species
In marine biology organisms the most common form of nitrogen waste is ammonia, whereas land-dwelling organisms convert the toxic ammonia to either urea or uric acid. Urea is found in the urine of and , as well as some fish. Birds and saurian reptiles have a different form of nitrogen metabolism that requires less water, and leads to nitrogen excretion in the form of uric acid. excrete ammonia, but shift to urea production during metamorphosis. Despite the generalization above, the urea pathway has been documented not only in mammals and amphibians, but in many other organisms as well, including birds, , insects, plants, yeast, fungi, and even .
Adverse effects
Urea can be irritating to skin, eyes, and the respiratory tract. Repeated or prolonged contact with urea in fertilizer form on the skin may cause dermatitis.
High concentrations in the blood can be damaging. Ingestion of low concentrations of urea, such as are found in typical human urine, are not dangerous with additional water ingestion within a reasonable time-frame. Many animals (e.g. Camel urine, rodents or dogs) have a much more concentrated urine which may contain a higher urea amount than normal human urine.
Urea can cause to produce toxins, and its presence in the runoff from fertilized land may play a role in the increase of toxic blooms.
The substance decomposes on heating above melting point, producing toxic gases, and reacts violently with strong oxidants, nitrites, inorganic chlorides, chlorites and perchlorates, causing fire and explosion.[ International Chemical Safety Cards: UREA. cdc.gov]
History
Urea was first discovered in urine in 1727 by the Dutch scientist Herman Boerhaave,[
Boerhaave called urea "sal nativus urinæ" (the native, i.e., natural, salt of urine). See:
]
-
The first mention of urea is as "the essential salt of the human body" in: Peter Shaw and Ephraim Chambers, A New Method of Chemistry …, vol 2, (London, England: J. Osborn and T. Longman, 1727), page 193: Process LXXXVII.
-
Boerhaave, Herman Elementa Chemicae …, volume 2, (Leipzig ("Lipsiae"), (Germany): Caspar Fritsch, 1732), page 276.
-
For an English translation of the relevant passage, see: Peter Shaw, A New Method of Chemistry …, 2nd ed., (London, England: T. Longman, 1741), page 198: Process CXVIII: The native salt of urine
-
Lindeboom, Gerrit A. Boerhaave and Great Britain …, (Leiden, Netherlands: E.J. Brill, 1974), page 51.
-
Backer, H. J. (1943) "Boerhaave's Ontdekking van het Ureum" (Boerhaave's discovery of urea), Nederlands Tijdschrift voor Geneeskunde (Dutch Journal of Medicine), 87 : 1274–1278 (in Dutch).
although this discovery is often attributed to the France chemist Hilaire Rouelle as well as William Cruickshank.[
]
Boerhaave used the following steps to isolate urea:
-
Boiled off water, resulting in a substance similar to fresh cream
-
Used filter paper to squeeze out remaining liquid
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Waited a year for solid to form under an oily liquid
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Removed the oily liquid
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Dissolved the solid in water
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Used recrystallization to tease out the urea
In 1828, the Germany chemist Friedrich Wöhler obtained urea artificially by treating silver cyanate with ammonium chloride.[Wöhler, Friedrich (1828) "Ueber künstliche Bildung des Harnstoffs" (On the artificial formation of urea), Annalen der Physik und Chemie, 88 (2) : 253–256. Available in English at Chem Team.
][
][
]
This was an organic compound was artificially synthesized from inorganic starting materials, without the involvement of living organisms. The results of this experiment implicitly discredited vitalism, the theory that the chemicals of living organisms are fundamentally different from those of inanimate matter. This insight was important for the development of organic chemistry. His discovery prompted Wöhler to write triumphantly to Jöns Jakob Berzelius:
In fact, his second sentence was incorrect. Ammonium cyanate and urea are two different chemicals with the same empirical formula , which are in chemical equilibrium heavily favoring urea under standard conditions.
Uremic frost was first described in 1865 by Harald Hirschsprung, the first Danish pediatrician in 1870 who also described the disease that carries his name in 1886. Uremic frost has become rare since the advent of Kidney dialysis. It is the classical pre-dialysis era description of crystallized urea deposits over the skin of patients with prolonged kidney failure and severe uremia.
Historical preparation
Urea was first noticed by Herman Boerhaave in the early 18th century from evaporates of urine. In 1773, Hilaire Rouelle obtained crystals containing urea from human urine by evaporating it and treating it with alcohol in successive filtrations.[Rouelle (1773) "Observations sur l'urine humaine, & sur celle de vache & de cheval, comparées ensemble" (Observations on human urine and on that of the cow and horse, compared to each other), Journal de Médecine, de Chirurgie et de Pharmacie, 40 : 451–468. Rouelle describes the procedure he used to separate urea from urine on pages 454–455.] This method was aided by Carl Wilhelm Scheele's discovery that urine treated by concentrated nitric acid precipitated crystals. Antoine François, comte de Fourcroy and Louis Nicolas Vauquelin discovered in 1799 that the nitrated crystals were identical to Rouelle's substance and invented the term "urea."[Fourcroy and Vauquelin (1799) "Extrait d’un premier mémoire des cit. Fourcroy et Vaulquelin, pour servir à l’histoire naturelle, chimique et médicale de l’urine humaine, contenant quelques faits nouveaux sur son analyse et son altération spontanée" (Extract of a first memoir by citizens Fourcroy and Vauquelin, for use in the natural, chemical, and medical history of human urine, containing some new facts of its analysis and its spontaneous alteration), Annales de Chimie, 31 : 48–71. On page 69, urea is named "urée".][Fourcroy and Vauqeulin (1800) "Deuxième mémoire: Pour servir à l’histoire naturelle, chimique et médicale de l’urine humaine, dans lequel on s’occupe spécialement des propriétés de la matière particulière qui le caractérise," (Second memoir: For use in the natural, chemical and medical history of human urine, in which one deals specifically with the properties of the particular material that characterizes it), Annales de Chimie, 32 : 80–112; 113–162. On page 91, urea is again named "urée".] Berzelius made further improvements to its purification
Laboratory preparation
Urea can be produced by heating ammonium cyanate to 60 °C.
Industrial production
In 2020, worldwide production capacity was approximately 180 million tonnes.
For use in industry, urea is produced from synthetic ammonia and carbon dioxide. As large quantities of carbon dioxide are produced during the ammonia manufacturing process as a byproduct of burning hydrocarbons to generate heat (predominantly natural gas, and less often petroleum derivatives or coal), urea production plants are almost always located adjacent to the site where the ammonia is manufactured.
Synthesis
The basic process, patented in 1922, is called the Bosch–Meiser urea process after its discoverers Carl Bosch and Wilhelm Meiser.
The second is urea conversion: the slower endothermic decomposition of ammonium carbamate into urea and water:
The overall conversion of and to urea is exothermic, with the reaction heat from the first reaction driving the second. The conditions that favor urea formation (high temperature) have an unfavorable effect on the carbamate formation equilibrium. The process conditions are a compromise: the ill-effect on the first reaction of the high temperature (around 190 °C) needed for the second is compensated for by conducting the process under high pressure (140–175 bar), which favors the first reaction. Although it is necessary to compress gaseous carbon dioxide to this pressure, the ammonia is available from the ammonia production plant in liquid form, which can be pumped into the system much more economically. To allow the slow urea formation reaction time to reach equilibrium, a large reaction space is needed, so the synthesis reactor in a large urea plant tends to be a massive pressure vessel.
Reactant recycling
Because the urea conversion is incomplete, the urea must be separated from the unconverted reactants, including the ammonium carbamate. Various commercial urea processes are characterized by the conditions under which urea forms and the way that unconverted reactants are further processed.
Conventional recycle processes
In early "straight-through" urea plants, reactant recovery (the first step in "recycling") was done by letting down the system pressure to atmospheric to let the carbamate decompose back to ammonia and carbon dioxide. Originally, because it was not economic to recompress the ammonia and carbon dioxide for recycle, the ammonia at least would be used for the manufacture of other products such as ammonium nitrate or ammonium sulfate, and the carbon dioxide was usually wasted. Later process schemes made recycling unused ammonia and carbon dioxide practical. This was accomplished by the "total recycle process", developed in the 1940s to 1960s and now called the "conventional recycle process". It proceeds by depressurizing the reaction solution in stages (first to 18–25 bar and then to 2–5 bar) and passing it at each stage through a steam-heated carbamate decomposer, then recombining the resulting carbon dioxide and ammonia in a falling-film carbamate condenser and pumping the carbamate solution back into the urea reaction vessel.
Stripping recycle process
The "conventional recycle process" for recovering and reusing the reactants has largely been supplanted by a stripping process, developed in the early 1960s by Stamicarbon in The Netherlands, that operates at or near the full pressure of the reaction vessel. It reduces the complexity of the multi-stage recycle scheme, and it reduces the amount of water recycled in the carbamate solution, which has an adverse effect on the equilibrium in the urea conversion reaction and thus on overall plant efficiency. Effectively all new urea plants use the stripper, and many total recycle urea plants have converted to a stripping process.
In the conventional recycle processes, carbamate decomposition is promoted by reducing the overall pressure, which reduces the partial pressure of both ammonia and carbon dioxide, allowing these gasses to be separated from the urea product solution. The stripping process achieves a similar effect without lowering the overall pressure, by suppressing the partial pressure of just one of the reactants in order to promote carbamate decomposition. Instead of feeding carbon dioxide gas directly to the urea synthesis reactor with the ammonia, as in the conventional process, the stripping process first routes the carbon dioxide through the stripper. The stripper is a carbamate decomposer that provides a large amount of gas-liquid contact. This flushes out free ammonia, reducing its partial pressure over the liquid surface and carrying it directly to a carbamate condenser (also under full system pressure). From there, reconstituted ammonium carbamate liquor is passed to the urea production reactor. That eliminates the medium-pressure stage of the conventional recycle process.
Side reactions
The three main side reactions that produce impurities have in common that they decompose urea.
Urea hydrolyzes back to ammonium carbamate in the hottest stages of the synthesis plant, especially in the stripper, so residence times in these stages are designed to be short.
Biuret is formed when two molecules of urea combine with the loss of a molecule of ammonia.
Normally this reaction is suppressed in the synthesis reactor by maintaining an excess of ammonia, but after the stripper, it occurs until the temperature is reduced.
Isocyanic acid HNCO and ammonia results from the thermal decomposition of ammonium cyanate , which is in chemical equilibrium with urea:
This decomposition is at its worst when the urea solution is heated at low pressure, which happens when the solution is concentrated for prilling or granulation (see below). The reaction products mostly volatilize into the overhead vapours, and recombine when these condense to form urea again, which contaminates the process condensate.
Corrosion
Ammonium carbamate solutions are highly corrosive to metallic construction materials – even to resistant forms of stainless steel – especially in the hottest parts of the synthesis plant such as the stripper. Historically corrosion has been minimized (although not eliminated) by continuous injection of a small amount of oxygen (as air) into the plant to establish and maintain a passive oxide layer on exposed stainless steel surfaces. Highly corrosion resistant materials have been introduced to reduce the need for passivation oxygen, such as specialized duplex stainless steels in the 1990s, and zirconium or zirconium-clad titanium tubing in the 2000s.
Finishing
Urea can be produced in solid forms (, granules, pellets or crystals) or as solutions.
Solid forms
For its main use as a fertilizer urea is mostly marketed in solid form, either as prills or granules. Prills are solidified droplets, whose production predates satisfactory urea granulation processes. Prills can be produced more cheaply than granules, but the limited size of prills (up to about 2.1 mm in diameter), their low crushing strength, and the caking or crushing of prills during bulk storage and handling make them inferior to granules. Granules are produced by acretion onto urea seed particles by spraying liquid urea in a succession of layers. Formaldehyde is added during the production of both prills and granules in order to increase crushing strength and suppress caking. Other shaping techniques such as pastillization (depositing uniform-sized liquid droplets onto a cooling conveyor belt) are also used.
Liquid forms
Solutions of UAN in water (UAN) are commonly used as a liquid fertilizer. In admixture, the combined solubility of ammonium nitrate and urea is so much higher than that of either component alone that it gives a stable solution with a total nitrogen content (32%) approaching that of solid ammonium nitrate (33.5%), though not, of course, that of urea itself (46%). UAN allows use of ammonium nitrate without the explosion hazard.
See also
External links
