Urea

Urea, also called carbamide, is a simple organic compound with the chemical formula CO(NH₂)₂. It is made up of two amino groups connected by a carbonyl group. Urea plays a big role in how animals process nitrogen in their bodies and is the main nitrogen-containing substance found in the urine of mammals.

This colorless, odorless solid dissolves easily in water and is not harmful. Our bodies use urea in many important processes, especially getting rid of extra nitrogen. It is made in the liver from ammonia and carbon dioxide through what is called the urea cycle.

Urea is very useful outside the body as well. It is commonly used in fertilizers to give plants the nitrogen they need to grow. In 1828, a scientist named Friedrich Wöhler discovered that urea could be made from non-living materials. This was a big moment in chemistry because it showed that substances from living things could be made in a lab, challenging old ideas about what could and could not be created from non-living material.

Molecular and crystal structure

Urea has a simple structure made up of oxygen, carbon, and nitrogen atoms. In solid form, the molecules line up in a flat way because of how the nitrogen atoms share their electrons. 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 is useful for separating 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 process can also affect proteins by adding weight to them. Because of this, it’s best to prepare fresh urea solutions for experiments and store them properly to keep them effective.

Analysis

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

Related compounds

Urea is a basic type of 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, larger organic pieces, or other types of groups. 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, more than 90% of the urea made around the world is used as a fertilizer to help plants grow. It provides nitrogen, a key nutrient that plants need. Urea works by breaking down in the soil into a form that plants can absorb through their roots. Sometimes, special treatments are used to control how quickly the nitrogen is released so it matches the plants' needs better.

Urea is also used to make certain types of plastics, especially those used in wood products like particleboard and plywood. In vehicles, urea helps clean up harmful gases from diesel engines by turning them into harmless nitrogen and water. It is mixed with water and stored in a special fluid called diesel exhaust fluid.

In hospitals and clinics, urea is found in creams that help keep skin moist and healthy. It is also used in special 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 process creates urea and carbon dioxide. The body removes the amino group from these acids and turns it into urea through a process called the urea cycle. Ammonia, which is made during this process, can be harmful, so many animals change it into urea to keep their bodies safe.

Urea is made in the liver and then travels through the blood to the kidneys, where 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, which can produce toxins. When urea from fertilizers runs off into water, it can help these toxic 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, showing that life’s building blocks could be created without living things. 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 harmful to plants but 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.