CH₄N₂O — Urea

Urea, chemically known as carbamide, is an organic compound with the molecular formula \(CH_4N_2O\). It is the simplest organic compound containing a carbonyl group (C=O) attached to two amine groups (–NH₂). Urea plays a crucial role in both biological and industrial processes. Biologically, it is the major nitrogen-containing compound in the urine of mammals, produced through the urea cycle to remove excess nitrogen from the body. Industrially, urea is widely manufactured as a fertilizer due to its high nitrogen content (about 46% by weight), making it the most commonly used nitrogen fertilizer worldwide.

Discovered by German chemist Friedrich Wöhler in 1828, urea became the first organic compound to be synthesized from inorganic materials. This landmark experiment disproved the vital force theory and marked the birth of modern organic chemistry.

The structure of urea consists of a central carbon atom double-bonded to an oxygen atom (carbonyl group) and bonded to two –NH₂ groups. The molecular formula is \(CO(NH_2)_2\). Its structural representation can be written as:

In the urea molecule, the carbon atom is sp²-hybridized, forming a planar structure. The carbon-oxygen double bond (C=O) and carbon-nitrogen bonds exhibit resonance, giving partial double-bond character to the C–N bonds. This resonance contributes to the stability of the molecule. The presence of multiple polar bonds and the ability to form hydrogen bonds with water make urea highly soluble in water and other polar solvents.

Urea is a natural metabolic byproduct formed in the liver of mammals during the urea cycle. This cycle converts toxic ammonia (NH₃), produced from protein and amino acid metabolism, into urea, which is then excreted through urine. The overall biochemical reaction for urea synthesis in the body can be represented as:

This process ensures the safe elimination of excess nitrogen from the human body. Besides its biological presence, urea is not found naturally in significant quantities in minerals or geological deposits. However, trace amounts may occur in animal excretions and soil as a result of microbial decomposition of proteins and other nitrogenous materials.

Industrial synthesis of urea is primarily achieved by the Bosch–Meiser process, which uses ammonia and carbon dioxide—both derived from the Haber–Bosch process. The process involves two main steps:

Step 1: Formation of Ammonium Carbamate

This reaction occurs under high pressure (150–250 bar) and temperature (170–200°C).

Step 2: Dehydration of Ammonium Carbamate

The resulting urea is then purified and granulated for use in agriculture and chemical industries. This process is energy-efficient and produces minimal waste, making it one of the most sustainable large-scale chemical manufacturing processes in the world.

Physical Properties:

• Appearance: White, crystalline solid with no characteristic odor.
• Solubility: Highly soluble in water and alcohol, insoluble in ether and chloroform.
• Hygroscopic: Urea absorbs moisture from the air easily.
• Decomposition: On heating, urea decomposes into ammonia, carbon dioxide, and biuret.

Chemical Properties:

• Hydrolysis: Urea hydrolyzes slowly in water and rapidly in the presence of the enzyme urease to form ammonia and carbon dioxide.

\(NH_2CONH_2 + H_2O \xrightarrow{urease} 2NH_3 + CO_2\)

• Reaction with Nitrous Acid: Urea reacts with nitrous acid to form nitrogen, carbon dioxide, and water.

\(NH_2CONH_2 + 2HNO_2 \rightarrow CO_2 + 2N_2 + 3H_2O\)

• Thermal Decomposition: When heated above 132°C, urea decomposes to form biuret and ammonia.

\(2NH_2CONH_2 \rightarrow NH_2CONHCONH_2 + NH_3\)

• Reaction with Aldehydes: Urea reacts with formaldehyde to form urea-formaldehyde resin, used in plastics and adhesives.

• Agriculture: Urea is the most widely used nitrogen fertilizer in the world, containing 46% nitrogen. It promotes plant growth and enhances crop yield. It is often applied directly to soil or as a foliar spray.
• Chemical Industry: Used as a raw material in the production of resins (urea-formaldehyde), melamine, and adhesives.
• Pharmaceuticals: Used in dermatological creams and ointments as a moisturizing agent and keratolytic compound to treat dry or rough skin conditions.
• Automotive Industry: Urea is used in diesel exhaust fluid (AdBlue) for Selective Catalytic Reduction (SCR) to reduce nitrogen oxide emissions.
• Laboratory Reagent: Used as a denaturant in protein biochemistry to unfold proteins and study their structure.

Urea’s versatility, low cost, and non-toxic nature have made it indispensable in agriculture, medicine, and environmental protection.

In living organisms, urea plays a critical role in the elimination of nitrogenous waste. It is synthesized in the liver through the urea cycle, which detoxifies ammonia produced during amino acid metabolism. The produced urea is transported via blood to the kidneys, where it is excreted in urine. The overall urea cycle reaction can be summarized as:

Urea concentration in blood (known as Blood Urea Nitrogen, BUN) is a vital diagnostic marker for kidney function. Abnormal levels of urea can indicate kidney disorders, dehydration, or liver dysfunction. Therefore, urea has both physiological and diagnostic significance.

Urea is generally non-toxic and safe to handle under normal conditions. However, excessive exposure to urea dust may cause mild skin or respiratory irritation. In agriculture, overuse of urea fertilizer can lead to environmental issues like soil acidification, eutrophication, and greenhouse gas emissions (N₂O). Controlled and balanced application is essential to minimize ecological impact.

In medicine, topical urea is beneficial for skin hydration and exfoliation, improving conditions such as eczema, psoriasis, and keratosis. Environmentally, urea’s use in diesel exhaust systems helps reduce harmful nitrogen oxide emissions, contributing to cleaner air.