Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. This amide has two amino groups (–NH2) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. Urea is New Latin, from French urée, from Ancient Greek οὖρον (ouron, "urine"), itself from Proto-Indo-European *h₂worsom.
More than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer.[8] 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. The most common impurity of synthetic urea is biuret, which impairs plant growth. Urea breaks down in the soil to give ammonium ions (NH+4). The ammonium is taken up by the plant through its roots. In some soils, the ammonium is oxidized by bacteria to give nitrate (NO−3), 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.
Urea is used in Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) reactions to reduce the NOx 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 (NH3) first produced by the hydrolysis of urea reacts with nitrogen oxides (NOx) and is converted into nitrogen gas (N2) and water within the catalytic converter. The conversion of noxious NOx to innocuous N2 is described by the following simplified global equation.
4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O
When urea is used, a pre-reaction (hydrolysis) occurs to first convert it to ammonia:
CO(NH2)2 + H2O → 2 NH3 + CO2
Being a solid highly soluble in water (545 g/L at 25 °C),[2] urea is much easier and safer to handle and store than the more irritant, caustic and hazardous ammonia (NH3), 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.
