Urea is one of the most important chemicals in use today - as a fertiliser and industrial raw material. It is also the chemical that gave birth to the science of organic chemistry.

Vitalism

Until the early 19th century, people - including many scientists - believed in a theory called vitalism. Those who believed in this theory held that life was not subject to the laws of physics and chemistry. They believed that there was an unknown, even divine principle, that governed living organisms, called the 'life spark'.

Because of this belief, it was thought that chemicals found in plant and animal bodies - like proteins and carbohydrates - were completely different from other chemicals like salts, acids and gases. Therefore, people thought that 'organic' chemicals (because they came from organs) could not be made artificially, but had to be extracted from living animals. This theory also stopped people from using inorganic chemicals to treat diseases.

Wohler synthesises urea

In 1773, French chemist Hilaire Rouelle discovered urea crystals from the urine of several animals, including humans. Chemical analysis showed that it was a very simple compound, which raised doubts about vitalism. Organic chemicals were known to be quite complex, and could not be made in the lab. But such a simple one could surely be?

The breakthrough came in 1828. And like many major discoveries in science, it was accidental. Friedrich Wohler (pictured) was a scientist at the polytechnic school of Berlin, who was then famous as the discoverer of aluminium. He was trying to make ammonium cyanate by mixing ammonium chloride with silver cyanate.

AgNCO + NH4Cl → NH4NCO + AgCl

But when he examined the resulting crystals closely, the compound he got did not behave like ammonium cyanate should. The crystals behaved like the ones Hilaire Rouelle had got - urea. Therefore Wohler came to a radical conclusion, as he wrote in a letter to the famous chemist Berzelius - I... must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea.

What actually happened was this. Ammonium cyanate was formed in the reaction, but it is an unstable compound. The atoms quickly rearrange themselves into urea, which is more stable. And so an inorganic compound became an organic one!

NH4NCO → H2N-CO-NH2

Interestingly, Wohler was upset by his discovery. He wrote to Berzelius, "The great tragedy of science, the slaying of a beautiful hypothesis by an ugly fact." For he saw Vitalism to be a beautiful concept, but the crystals of urea seemed to say that the theory was false.

Organic chemistry

Though Wohler's urea synthesis is famous, it was not the first organic compound to be made in the lab. In fact Wohler himself had made oxalic acid from an inorganic compound called cyanogens.

There was a huge amount of resistance to the idea that vitalism wasn't correct. Indeed Wohler himself did not like it. Influential scientists like Justus von Liebig and Louis Pasteur weren't convinced either. Many organic compounds still could not be made in the lab at all, from inorganic ones. (Even today, some very complicated molecules like insulin cannot be made in the lab without using living organisms.) The tide changed only in 1845, when Hermann Kolbe showed that carbon disulfide could be converted to acetic acid, the main ingredient of vinegar.

But meanwhile a whole lot of scientists saw the practical uses of Wohler's discovery. For many organic chemicals like urea (till then obtained from kidneys), citric acid (obtained from lemons) and benzene (obtained from gum benzoin) were industrially very useful. If they could be made from inorganic chemicals, then they could be made cheaper and on a large scale.

Soon a huge industry had sprung up, with synthetic dyes (see the articles on Perkin and indigo) and drugs (see the article on salvarsan) being made on a large scale. Today, organic chemistry makes more than a million chemicals every year!

History

Urea was first discovered in urine in 1727 by the Dutch scientist Herman Boerhaave, though

this discovery is often attributed to the French chemistHilaire Rouelle.[3] In 1828,

the German chemist Friedrich Wöhler obtained urea by treating

silver isocyanate with ammonium chloride.[4][5][6]

AgNCO + NH4Cl → (NH2)2CO + AgCl

This was the first time an organic compound was artificially synthesized from inorganic

starting materials, without the involvement of living organisms. The results of this

experiment implicitly discredited vitalism: the theory that the chemicals of living

organisms are fundamentally different from inanimate matter. This insight was important

for the development of organic chemistry. His discovery prompted Wöhler to write

triumphantly toBerzelius: "I must tell you that I can make urea without the use of kidneys,

either man or dog. Ammonium cyanate is urea." For this discovery, Wöhler is considered

by many[who?] the father of organic chemistry.

Laboratory uses

Urea in concentrations up to 10 M is a powerful protein denaturant as it disrupts the

noncovalent bonds in the proteins. This property can be exploited to increase the solubility of

some proteins. A mixture of urea and choline chloride is used as a deep eutectic solvent, a

type of ionic liquid.

Urea can in principle serve as a hydrogen source for subsequent power generation in fuel

cells. Urea present in urine/wastewater can be used directly (though bacteria normally quickly

degrade urea.) Producing hydrogen by electrolysis of urea solution occurs at a lower voltage

(0.37V) and thus consumes less energy than the electrolysis of water (1.2V).[13]

Urea in concentrations up to 8 M can be used to make fixed brain tissue transparent to visible

light while still preserving fluorescent signals from labeled cells. This allows for much deeper

imaging of neuronal processes then previously obtainable using conventional one photon or

two photon confocal microscopes.[14]

Medical use

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"[15] of the

nail plate. Only diseased or dystrophic nails are removed, as there is no effect on healthy

portions of the nail. This drug is also used as an earwax removal aid. Urea can also be used

as a Diuretic.

Certain types of instant cold packs (or ice packs) contain water and separated urea crystals.

Rupturing the internal water bag starts an endothermic reaction and allows the pack to be

used to reduce swelling.

Like saline, urea injection is used to perform abortions.

Urea is the main component of an alternative medicinal treatment referred to as urine therapy.

The blood urea nitrogen (BUN) test is a measure of the amount of nitrogen in the blood that

comes from urea. It is used as a marker of renal function.

Urea labeled with carbon-14 or carbon-13 is used in the urea breath test, which is used to

detect the presence of the bacteria Helicobacter pylori (H. pylori) in

the stomach and duodenum of humans, associated with peptic ulcers. The test detects the

characteristic enzyme urease, produced by H. pylori, by a reaction that produces ammonia

from urea. This increases the pH (reduces acidity) of the stomach environment around the

bacteria. Similar bacteria species to H. pylori can be identified by the same test in animals

such as apes, dogs, and cats (including big cats).