Urea, also known as carbamide, is an organic compound with the chemical formula CO(NH2)2. This amide has two –NH2 groups joined by a carbonyl (C=O) functional group.
In this post, Pritish Halder explains synthesis, explosiveness and different uses of Urea
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. It is a colorless, odorless solid, highly soluble in water, and practically non-toxic (LD50 is 15 g/kg for rats). Dissolved in water, it is neither acidic nor alkaline.
The body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two ammonia molecules (NH3) with a carbon dioxide (CO2) molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen (N) and is an important raw material for the chemical industry.
Friedrich Wöhler discovered that urea can be produced from inorganic starting materials, which was an important conceptual milestone in chemistry in 1828. It showed for the first time that a substance previously known only as a byproduct of life could be synthesized in the laboratory without biological starting materials, thereby contradicting the widely held doctrine of vitalism, which stated that only living things could produce the chemicals of life.
The basic process, developed in 1922, is also called the Bosch–Meiser urea process after its discoverers. Various commercial urea processes are characterized by the conditions under which urea forms and the way that unconverted reactants are further processed. The process consists of two main equilibrium reactions, with incomplete conversion of the reactants. The first is carbamate formation: the fast exothermic reaction of liquid ammonia with gaseous carbon dioxide (CO2) at high temperature and pressure to form ammonium carbamate ([H2N−CO2][NH4])
2 NH3 + CO2 ⇌ [H2N−CO2][NH4] (ΔH = -117 kJ/mol at 110 atm and 160 °C)
The second is urea conversion: the slower endothermic decomposition of ammonium carbamate into urea and water:
[H2N−CO2][NH4] ⇌ (NH2)2CO + H2O (ΔH = +15.5 kJ/mol at 160 – 180 °C)[5
More 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 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. The ammonium is taken up by the plant. In some soils, the ammonium is oxidized by bacteria to give nitrate, which is also a plant nutrient.
Urea is a raw material for the manufacture of two main classes of materials: urea-formaldehyde resins and urea-melamine-formaldehyde used in marine plywood.
Urea can be used to make urea nitrate, a high explosive that is used industrially and as part of some improvised explosive devices.
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. The ammonia produced by the hydrolysis of the urea reacts with the nitrogen oxide emissions and is converted into nitrogen and water within the catalytic converter. Trucks and cars using these catalytic converters need to carry a supply of diesel exhaust fluid, a solution of urea in water.
Urea-containing creams are used as topical dermatological 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” of the nail plate. Only diseased or dystrophic nails are removed, as there is no effect on healthy portions of the nail . This drug (as carbamide peroxide) is also used as an earwax removal aid.