Urea, also known as carbamide, is an organic nitrogen containing compound with a chemical formula of CO(NH2)2. It is often considered to be the chemical that gave birth to organic chemistry (1). Until the early 19th century most people believed in the theory of vitalism, or the belief that life was not subject to the laws of physics or chemistry but that life contained some divine principle that they called the “life spark”. This caused the belief that any chemical found in any living thing, such as proteins and carbohydrates, were not like any of the non-living chemicals (1). With a relatively primitive understanding of chemistry at this time, scientists did not believe that “organic” compounds could be synthesized and that they must be obtained naturally from a living source. The counter-evidence to the vitalism theory came with the discovery and eventual synthesis of urea. Urea was discovered in 1727 by the Dutch physician Herman Boerhaave (2). Boerhaave isolated urea by purifying urine samples from several animals (1). Unfortunately Boerhaave saw himself as a physician, and rightfully so as he is now considered the founder of clinical teaching, and not a chemist and therefore was reluctant to publish his findings. He even was quoted as saying “Nothing was formerly further from my thoughts than that I should trouble the world with anything in chemistry” (3). Eventually Boerhaave did publish his findings, but only after his students published it on his behalf. However because Boerhaave was not concerned with chemistry, these findings were seen by very few and were ultimately forgotten. Some people accredit the discovery of urea to the French chemist Hilaire Rouelle in 1773, 50 years after Boerhaave (2). Gradually Boerhaave is being given the credit he has earned for his discovery. Urea was actually named in 1797 by the French chemists Fourcroy and Vauquelin.
In mammals urea is naturally synthesized in the liver as part of the urea cycle, either from the oxidation of amino acids or from ammonia. Amino acids are brought into the body through the breakdown of foods. Amino acids are mostly used during the synthesis of peptides and proteins, but any excess can be metabolized to produce a small amount of energy (3). Ammonia is a byproduct of the metabolizing of nitrogenous compounds and the buildup of ammonia can raise the pH of cells to toxic levels (2). Because of this potential toxicity problem, the human body will spend energy to convert ammonia to urea which is practically harmless and can be removed through the urine or through sweat. Although urea itself is colorless and odorless, it readily decomposes in water back to ammonia, which is why urine has a characteristic smell. The continued decomposition of the urea in urine is the reason why stale urine is so much more pungent than fresh urine (3). Urea can also be produced industrially through a variety of chemical processes.
Urea was first synthesized by the German chemist Friedrich Wöhler in 1828. Wöhler was able to obtain urea by treating silver cyanate with ammonium chloride:
AgNCO + NH4Cl → (NH2)2CO + AgCl
The creation of urea, an organic compound, from inorganic reactants severely discredited the theory of vitalism. For this discovery, Wöhler is considered by many to be the father of organic chemistry (2). Ultimately this process was not practical for an industrial scale. There are several modern methods of producing urea. The most common is the Bosch-Meiser urea process (2). This process has two major steps: the fast exothermic reaction of liquid ammonia with gaseous carbon dioxide at high temperature and pressure to form ammonium carbamate:
2NH3 + CO2 H2N-COONH4
And the second step in the process – the endothermic decomposition of ammonium carbamate into urea and water:
H2N-COONH4 (NH2)2CO + H2O
Modern technology allows for the almost total recycling of unused carbon dioxide and ammonia. Overall the Bosch-Meiser process is exothermic. The process supplies a majority of its own heat as the heat given off from the first reaction can be used to drive the second reaction (2). Strangely enough the conditions for one step of this process is very unfavorable for the other step, so this process is done through an environmental compromise. The high temperature (190ºC) needed for the second step is compensated for by conducting the process at a high pressure (140-175 bar), which favors the first reaction (2). Due to the length of time that is required for the decomposition of carbamate into urea to reach an equilibrium, synthesis reactors tend to be enormous pressure vessels. The slow urea conversion reaction has the potential for two side reactions that produce impurities. Biuret is formed when two molecules of urea combine with the loss of a molecule of ammonia (2):
2NH2CONH2 → H2NCONHCONH2 + NH3
This side reaction is generally avoided by maintaining an excess of ammonia in the synthesis reactor. Biuret is undesirable in urea fertilizer because it is toxic to some plants. However, it is actually preferred in cattle feed. The second impurity, called isocyanic acid, can result from the decomposition of ammonium cyanate, which is in equilibrium with urea.
NH2CONH2 → NH4NCO → HNCO + NH3
This reaction occurs when the urea solution is heated at low pressure, such as when the solution is being concentrated (2). The reaction products volatilize into vapors and then recombine when everything is condensed into urea, ultimately contaminating the process condensate. Urea can also be made on a relatively small scale in laboratories through the reaction of phosgene with ammonia (2).
COCl2 + 4 NH3 → (NH2)2CO + 2 NH4Cl
Urea has a chemical formula of CO(NH2)2, a molecular weight of 60.05526 g/mol, and a density of 1.323 g/cm3. It is usually found as white crystals or powder. Urea may gradually develop an odor if any moisture reacts with it as it will gradually decompose back into ammonia. It has a melting point of 132.7ºC and no boiling point because it decomposes before it boils. Urea is insoluble in benzene, soluble in concentrated HCl and pyrimidine, and is soluble up to 545 g/L in water at 25ºC. A 10% urea solution has a pH of 7.2. Urea has a vapor pressure of 1.2×10-5 mmHg at 25ºC (4).
In 2012 approximately 184 million tons of urea was produced through industrial processes. More than 90% of this was used in the production of nitrogen-release fertilizers (2). Urea has the highest nitrogen content of all solid nitrogen fertilizers that are in use. Many of the bacteria in soil have the enzyme urease, which is used to catalyze the decomposition of urea into ammonia, or ammonium and bicarbonate ions. Ammonium coupled with nitrate are the major sources of nitrogen for plant growth. This allows for much better crop yields from an area than would be allowed by nature. Urea can also be used as a small supplement to cattle feed. Urea is also used in the chemical industry as a raw material for urea-formaldehyde resins, which are often used during the production of plywood. Urea can also be used to make urea nitrate which is a high explosive that is used industrially, but is also a common ingredient in some improvised explosive devices. It can also be used in selective catalytic reduction (SCR) and non-selective catalytic reduction (NSCR) reactions in automobiles to reduce the various nitrogen oxide pollutants in the exhaust from diesel engines (2). In chemical and medical laboratories urea can: be used as an agent to denature proteins, serve as a hydrogen source for fuel cells, and to make fixed brain tissue transparent to visible light while still preserving fluorescent signals from specific labeled cells (2). Urea-containing creams can be used for medical purposes like the rehydration of the skin and debridement of nails. Strangely enough urea can also be used as a diuretic that is safe and inexpensive. Urea has niche uses as an ingredient in dish soap, a flavor enhancer for cigarettes, an additive to dye baths, and a myriad of other uses (2).
The major strategic importance of urea is its use as a fertilizer. Naturally plants need ammonium to grow and ammonium is not overwhelmingly prevalent in most soils. Nitrogen fertilizers, with the fertilizer with the highest nitrogen content being urea based, introduce a method to increase the ammonium potential of the soil. This artificially produces relatively rich soil with an increased capability to make crops grow. The increased levels of nitrogen in the soil from the fertilizers and the bacteria that transform it into ammonium allow for better crop yields in a smaller area (5). This breakthrough in artificial nitrogen fixation has allowed the Earth to produce more food than would naturally be possible, thereby increasing its carrying capacity. Any country that in any way grows a moderate amount of food should consider urea to be important.
The current price for urea is $297 per metric ton. The price of urea has been on a very gradual decline since it reached its peak in September 2011 at $503.80, with the exception of a spike in price in the spring of 2012 where it peaked at $496.70 in May (6). This trend appears to be predicted to continue as production prices continue to decrease as technology continues to advance.
The MSDS fact sheet for urea states that it has a mutagenic effect on mammalian somatic cells and that prolonged exposure to it can produce organ damage. In case of eye contact, wash eye for at least 15 minutes and seek medical attention. In case of skin contact be sure to flush the skim with plenty of water and cover the irritated skin with an emollient and seek medical attention. If inhaled, move to fresh air or give oxygen and seek medical attention. If urea is ingested it is recommended that you loosen any tight clothing and only get medical attention if symptoms appear. Urea may be combustible at high temperatures. Storage for urea involves keeping it away from heat or any source of ignition. When handling urea it is recommended to wear splash goggles, a lab coat, dust respirator, and gloves (7).
Urea can be analyzed through a variety of methods. It has been shown that thin-layer chromatography (TLC) is a feasible means of determining the presence of urea in solution (4). Urea can also be analyzed via IR spectrophotometry and LC-MS. Urea is often used in conjunction with LC-MS in bottom-up proteomics as a denaturing agent for proteins (8).
Only having immediate access to the information, and thus the viewpoints, of today greatly influences the perceived importance of a substance. Having a 21st century bias in determining the ultimate significance of something that is relatively common is difficult, however, urea can be considered a molecule that has effected cultures in a positive way. Simply put urea, with its ability to be made into fertilizer and maintain its high nitrogen content, has allowed for the growth of not only the populations of individual countries, but the world as a whole. The widely used and accepted use of large scale “industrial” farms is reliant on nitrogen based fertilizers to maintain their level of output. Without fertilizer, the natural nitrogen content of the soil would not be enough to produce the quantity or quality of crops that are now available. For this standpoint it can be seen that urea, through efficient fertilization, has effected culture by allowing the continuation of large scale growth. As countries continue to develop and cities continue to expand, more and more food is ultimately required to be grown to allow for this localization of population. In 2013 China produced 46 million tons of urea, almost one third of the total urea produced that year (9). The Chinese dominance in urea production does not necessarily mean that the Chinese benefit the most from it. Arguably, any nation that produces a sizable about of food with a good crop yield to acreage ratio can be assumed to have benefitted from urea. This may include the United States, China, India, Brazil, and Russia (10).
Ultimately urea has changed society as a whole. Without the ability to introduce additional nitrogen into the soil artificially, the carrying capacity of the earth would be much lower than it is presently. Although not incredibly rare or valuable from a monetary standpoint, urea has made an incredible impact on society that is often overlooked. It has increased crop yields by 30%-50% versus fields not treated with urea fertilizer (11). This has allowed for less land being required for food production and opened up more land for urban development.
References
(1). http://humantouchofchemistry.com/urea-and-the-beginnings-of-organic-chemistry.htm , accessed 8 Mar, 2015.
(2). http://en.wikipedia.org/wiki/Urea , accessed 8 Mar, 2015.
(3). http://www.rsc.org/chemistryworld/podcast/CIIEcompounds/transcripts/urea.asp , accessed 8 Mar, 2015.
(4). http://pubchem.ncbi.nlm.nih.gov/compound/urea#section=Solubility , accessed 8 Mar, 2015.
(5). http://www.education.com/science-fair/article/effect-physical-form-fertilizer-plant/ , accessed 8 Mar, 2015.
(6). http://www.indexmundi.com/commodities/?commodity=urea&months=60 , accessed 8 Mar, 2015.
(7). http://www.sciencelab.com/msds.php?msdsId=9927317 , accessed 8 Mar, 2015.
(8). http://en.wikipedia.org/wiki/Liquid_chromatography%E2%80%93mass_spectrometry#Proteomics.2Fmetabolomics , accessed 9 Mar, 2015.
(9). http://minerals.usgs.gov/minerals/pubs/commodity/nitrogen/mcs-2014-nitro.pdf , accessed 9 Mar, 2015.
(10). http://en.wikipedia.org/wiki/List_of_largest_producing_countries_of_agricultural_commodities , accessed 9 Mar, 2015.
(11). http://www.ipni.net/ppiweb/ppinews.nsf/$webcontents/7DE814BEC3A5A6EF85256BD80067B43C/$file/Crop+Yield.pdf , accessed 9 Mar, 2015.