Urea is synthesized in the liver and is a nitrogen-containing metabolite excreted by mammals. This metabolic process is called urea cycle.
Urea is the first organic compound obtained by artificial synthesis of inorganic substances. The theory of vitality was overthrown.
Alias: carbonyl diamine, carboxamide, urea
physiology
Urea is produced in the liver and mixed into the blood (the concentration in the human body is between 2.5 and 7.5 micromoles per liter), and finally excreted from the urine through the kidney. A small amount of urea is excreted through sweat.
Biology uses carbon dioxide, water, aspartic acid and ammonia to synthesize urea. The metabolic pathway to promote urea synthesis is a kind of synthetic metabolism, which is called urea cycle. This process consumes energy, but it is necessary. Because ammonia is toxic and a common metabolite, it must be eliminated. N- acetylglutamic acid is needed as a regulator when the liver synthesizes urea.
Nitrogen-containing waste is toxic, which is produced by protein and amino acid catabolism (deamination is the process of deamination of amino acids, and the nitrogen-containing compounds produced in this process are converted into urea in the liver, and the nitrogen-free parts are converted into sugars or fats, etc. ). Most living things have to be reprocessed. Marine organisms are usually discharged directly into seawater in the form of ammonia. Terrestrial organisms convert ammonia into urea or uric acid, and then excrete it. Birds and reptiles usually excrete uric acid while other animals (such as mammals) excrete urea. The exception is that, for example, aquatic tadpoles excrete ammonia, but during metamorphosis, they excrete urea; Damamachi dogs mainly excrete uric acid, not urea, because the gene of an invertase in the urea cycle is destroyed.
Mammals produce urea through a circulatory reaction in the liver. This cycle was first proposed in 1932, and its reaction starting point is the decomposition of ammonia. After 1940s clarified the function of citrulline and arginine succinic acid, people fully understood it. In this cycle, ammonia and amino groups of L- aspartic acid are converted into urea, and L- ornithine, citrulline, L- arginine-succinic acid and L- arginine are used as media.
Urea cycle is the main way for mammals and amphibians to excrete nitrogen-containing metabolic wastes. But so do other organisms, such as birds, invertebrates, insects, plants, yeast, fungi and microorganisms.
Urea is basically a waste to living things, but it still has positive value. For example, urea in renal tubules is introduced into renal cortex to increase its osmotic concentration, so that water can permeate back into the body from renal tubules for reuse.
chemical property
Molecular formula: CO(NH2)2, molecular weight 60.06, colorless or white needle-like or rod-like crystals of CO(NH2)2, white reddish solid particles, used in industrial and agricultural products, odorless and tasteless. The density is1.335g/cm3. The melting point is 132.7℃. Soluble in water and alcohol, insoluble in ether and chloroform. Slightly alkaline. Can react with acids to form salts. There is hydrolysis. The condensation reaction can be carried out at high temperature to produce biuret, biuret and cyanuric acid. Heat to 65438+/-060℃ for decomposition, and that generated ammonia gas is simultaneously convert into cyanic acid. Because human urine contains this substance, it is named urea. Urea contains N)46%% nitrogen (N), which is the highest among solid nitrogen fertilizers.
Production method: urea is directly synthesized from ammonia and carbon dioxide at high temperature and high pressure. The chemical reaction is as follows.
Urea can be hydrolyzed to produce ammonia and carbon dioxide under the action of acid, alkali and enzyme (acid and alkali need to be heated).
Unstable to heat, it will deamination into biuret when heated to 150~ 160℃. If heated rapidly, it will deamino and trimerize into a six-membered ring compound cyanuric acid. (Mechanism: first deamination to produce isocyanate (HN=C=O), and then trimerization. )
It can react with acetyl chloride or acetic anhydride to produce acetourea and diacetyl urea.
Under the action of sodium ethoxide, it reacts with diethyl malonate to form malonyl urea (also called barbituric acid, because it has certain acidity).
Under the action of alkaline catalysts such as ammonia water, it can react with formaldehyde and condense to produce urea-formaldehyde resin.
React with hydrazine hydrate to produce semicarbazide.
2NH3+CO2 → NH2coonh4 → Co (NH2) 2+H2O Urea is easily soluble in water, and 105g can be dissolved in 100 ml water at 20℃, and the aqueous solution is neutral. There are two kinds of urea products. Crystallized urea is a white needle-like or prismatic crystal with strong hygroscopicity. Granular urea is translucent, with a particle size of 1 ~ 2 mm, smooth in appearance and obviously improved in hygroscopicity. At 20℃, the critical moisture absorption point is 80% of the relative humidity, but at 30℃, it drops to 72.5%. Therefore, urea should be avoided to be stored in the open air in humid climate in midsummer. At present, hydrophobic substances such as paraffin wax are added in urea production, which greatly reduces the hygroscopicity.
Urea is a kind of high concentration nitrogen fertilizer, which belongs to neutral quick-acting fertilizer and can also be used to produce a variety of compound fertilizers. No harmful substances remain in the soil, and long-term application has no adverse effects. Animal husbandry can be used as feed for ruminants. However, when the granulation temperature is too high, a small amount of biuret, also known as biuret, will be produced, which will inhibit crops. China stipulates that the biuret content of urea for fertilizer should be less than 0.5%. When the biuret content exceeds 1%, it cannot be used as seed fertilizer, seedling fertilizer and foliar fertilizer, and the urea content in other application periods should not be too high or too concentrated.
Urea is an organic nitrogen fertilizer, which can only be absorbed and utilized by crops after being hydrolyzed into ammonium carbonate or ammonium bicarbonate by urease in soil. Therefore, urea should be applied 4 ~ 8 days before the fertilizer requirement period of crops.