اليوريا

اليوريا مادة كيميائية بلورية بيضاء، غير متطايرة، عديمة الرائحة نسبيًّا، وذو نقطة انصهار مرتفعة تبلغ حوالي 133°م، وتذوب بسهولة في الماء والكحول. وهي تصنَّف كأميد عضوي بسيط، وتُعدُّ من أهم المركبات النيتروجينية العضوية الصناعية من حيث الحجم العالمي للإنتاج. تُستخدم اليوريا على نطاق واسع كمصدر رئيسي للنيتروجين في الأسمدة الزراعية، كما تلعب دورًا حيويًّا كمادة وسيطة في تصنيع الراتنجات الأمينية مثل راتنجات الفورمالديهايد-اليوريا، والتي تُطبَّق في صناعات الأخشاب المضغوطة والطلاءات واللدائن. كما تدخل في تركيب بعض الأدوية والمواد المضافة في صناعة الأغذية، فضلًا عن استخداماتها الناشئة في أنظمة خفض انبعاثات أكاسيد النيتروجين في محركات الديزل عبر تقنية SCR.

Agriculture usesMore than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has the lowest transportation costs per unit of nitrogen nutrient.In the soil, it hydrolyses back to ammonia and carbon dioxide. The ammonia is oxidized by bacteria in the soil to nitrate, which can be absorbed by the plants. Urea is also used in many multi-component solid fertilizer formulations. Urea is highly soluble in water, therefore, very suitable for use in fertilizer solutions (in combination with ammonium nitrate: UAN), e.g., in ‘foliar feed’ fertilizers. For fertilizer use, granules are preferred because of their narrower particle size distribution, an advantage for mechanical application. The most common impurity of synthetic urea, biuret, must be present at less than 2 percent of the time, as it impairs plant growth.
The urea molecule is planar in the crystal structure, but the geometry around the nitrogens is pyramidal in the gas-phase minimum-energy structure. In solid urea, the oxygen center is engaged in two N-H-O hydrogen bonds. The resulting dense and energetically favourable 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 basic compared to, say, formaldehyde. Urea's high aqueous solubility reflects its ability to engage in extensive hydrogen bonding with water.Urea dissolved in water is in equilibrium with the isomeric ammonium cyanate. The resulting activity of the isocyanic acid ions do result in carbamylation (formation of long-chain carbamides, liberating ammonia molecule as byproduct) of proteins if proteins are present in the solution too. The carbamylation reaction may occurs at elevated temperatures even without catalysts. At room temperature, water solutions of urea are prone to same decomposition reaction in the presence of urease. The isomerization of urea in solution at room temperature without catalysts is a slow process (taking days to reach equilibrium), and freshly prepared, unheated solutions had negligible carbamylation rates.Urea can react with alcohols to form urethanes and react with malonic esters to make barbituric acids.