Urea

Urea, also called carbamide, is a simple organic compound with the chemical formula CO(NH₂)₂. It has two amino groups connected by a carbonyl group. Urea helps animals get rid of nitrogen and is the main nitrogen-containing substance in the urine of mammals.

This white, odorless solid dissolves easily in water and is safe. Our bodies use urea in many important ways, especially to remove extra nitrogen. It is made in the liver from ammonia and carbon dioxide in a process called the urea cycle.

Urea is also useful outside the body. It is often used in fertilizers to help plants grow. In 1828, a scientist named Friedrich Wöhler discovered that urea could be made from non-living materials. This was important in chemistry because it showed that substances from living things could be made in a lab.

Molecular and crystal structure

Urea has a simple structure made of oxygen, carbon, and nitrogen atoms. In solid form, the molecules line up in a flat way. But when it’s a gas or mixed with water, the shape changes slightly.

Urea can form special structures that trap other molecules inside. These structures are made of urea molecules linked together, creating spaces where other tiny particles can fit. This property helps separate different types of molecules.

Reactions

Basicity

Urea can act like a weak base. When it mixes with strong acids, it changes to form special salts called uronium salts. It can also connect with metals to make complex shapes.

N-functionalization

Urea can change easily when mixed with certain chemicals. This helps create new useful substances. For example, it can turn into N-nitrourea when treated in a special way.

Transamination

Urea can also mix with other chemicals to make new compounds. One example is making N-phenylurea when treated with anilinium.

Heterocyclization

Urea is helpful for making different ring-shaped molecules. It can combine with other substances to create barbituric acids and pyrimidines, among other things.

Thermolysis

When urea is heated, it breaks down into simpler parts. At around 152 °C (306 °F), it turns into ammonium cyanate. If heated more, above 160 °C (320 °F), it changes into ammonia and isocyanic acid.

Hydrolysis

In water, urea slowly changes into ammonium cyanate. This can also affect proteins. Because of this, it’s best to prepare fresh urea solutions for experiments and store them properly.

Analysis

Urea can be measured using different methods, such as the diacetyl monoxime colorimetric method and the Berthelot reaction. These methods work well with special machines for quick testing.

Related compounds

Urea is a basic chemical that helps form a group of similar chemicals. These chemicals all have a special part called a carbonyl group connected to two amine groups. These groups can be simple hydrogen atoms or larger organic pieces. Some examples of these related chemicals are carbamide peroxide, allantoin, and hydantoin. Ureas are also linked to other chemical families like biurets, carbamates, carbodiimides, and thiocarbamides.

Uses

Urea has many important uses in different areas.

In farming, most urea is used as a fertilizer to help plants grow. It gives plants nitrogen, a nutrient they need. Urea breaks down in the soil so plants can take it up through their roots.

Urea is also used to make plastics for wood products like particleboard and plywood. In cars, urea helps clean gases from diesel engines, turning them into harmless nitrogen and water. It is mixed with water and stored as a special fluid called diesel exhaust fluid.

In hospitals, urea is in creams that keep skin moist. It is also used in tests to find bacteria in the stomach that can cause ulcers.

Physiology

When we eat food, our bodies can use amino acids for energy. This makes urea and carbon dioxide. The body changes the amino group into urea through a process called the urea cycle. Ammonia, which is made during this process, can be harmful, so animals change it into urea to stay safe.

Urea is made in the liver and then moves through the blood to the kidneys. It is removed from the body in urine. It helps the body manage water and keep the right balance of salts in the blood. Different animals handle waste in different ways—some fish release ammonia right away, while mammals and amphibians turn it into urea. Birds and some reptiles create uric acid instead, which uses less water.

Adverse effects

Urea can irritate the skin, eyes, and breathing passages. If it touches the skin a lot, it might cause redness or rash.

High levels of urea in the blood can be harmful. Small amounts of urea, like those found in human urine, are not dangerous if you drink water soon after. Some animals, such as camels, rodents, and dogs, have urine with more urea than humans.

Urea can also lead to harmful growths in water called algal blooms. When urea from fertilizers runs off into water, it can help these blooms grow.

When heated, urea breaks down and can release harmful gases. It also reacts strongly with certain chemicals, which can cause fires or explosions.

History

Urea was first found in 1727 by Herman Boerhaave from dried urine. Later, French chemist Hilaire Rouelle and William Cruickshank also helped discover it. In 1773, Rouelle made crystals of urea by drying human urine and adding alcohol.

In 1828, German chemist Friedrich Wöhler made urea from simple chemicals. This helped change our understanding of chemistry.

Laboratory preparation

Urea can be made by heating ammonium cyanate to 60 °C (140 °F).

The process follows this reaction:

[NH₄]⁺[OCN]⁻ → (NH₂)₂CO

Industrial production

In 2020, the world made about 180 million tonnes of urea for different uses.

Urea is made from two substances: synthetic ammonia and carbon dioxide. When making ammonia, we also get carbon dioxide as a by-product from burning fuels like natural gas. Because of this, urea factories are usually built next to ammonia factories.

Synthesis

The way we make urea was discovered in 1922 by Carl Bosch and Wilhelm Meiser. It has two main steps.

First, we mix liquid ammonia with carbon dioxide gas under high temperature and pressure. This makes ammonium carbamate.

Second, ammonium carbamate changes into urea and water. This step is slower.

Both steps happen at the same time in big, strong containers called reactors.

Reactant recycling

Because not all the ammonia and carbon dioxide turn into urea, we need to reuse what’s left. There are two main ways to do this.

In older methods, we let the pressure drop to get the ammonia and carbon dioxide back. Now, most factories use a method called “stripping,” which keeps the pressure high and makes the process simpler.

Side reactions

Sometimes, urea can change back into other substances. One of these is called biuret, which can be useful for animal food. Another is isocyanic acid, which can cause problems during processing.

Corrosion

The mixtures used to make urea can damage metal parts of the factory, especially in hot areas. Special metals and methods help prevent this damage.

Finishing

Urea can be made into solid pieces called prills or granules, or into liquid solutions. Solid forms are mostly used as fertilizer. Liquid urea mixed with ammonium nitrate is also used as a fertilizer and is safer than using ammonium nitrate alone.