Skip to content

Latest commit

 

History

History
133 lines (119 loc) · 7.17 KB

2010-04-20_eight-molecules-that-changed-the-rules-of-the-game-urea.rst

File metadata and controls

133 lines (119 loc) · 7.17 KB
Raw

Eight molecules that changed the rules of the game: Urea

Author: Stefano
category:Chemistry
tags:molecules, urea

Rule changed: demonstrated that organic compounds had no mysterious "vital energy"

image

The synthesis of urea is a fascinating and critical event. It sent a shocking quake through many open questions in chemistry, and answered them with a cold hard fact with no chance of misunderstanding. It slain one theory, vitalism, and was in front line to promote the discipline of "organic chemistry", the chemistry of carbon compounds.

Since the discovery of fire, a useful taxonomy of substances was "those which burn" versus "those which melt". The freezing caveman probably learned rather quickly to bring wood instead of rocks in his cave. Two important additional observation that involved chemists and philosophers alike were that everything that burned came from living creatures, and everything that burned could not be restored to its original form once the heat source was removed. Today, we know they decompose, but at that time, the main doctrine was that heat forever removed some special substance, a "vital force", whose intrinsic characteristic was to grant life and life-related behavior. The vitalistic theory was clear: you could get from vital matter to non-vital matter, but you could never do the opposite, i.e. create vital force from nothing. Vital, organic substances could only be created by the living bodies of animals and plants.

`Wöhler <http://en.wikipedia.org/wiki/Friedrich_W%C3%B6hler>`_, a german chemist born in 1800, was the man who destroyed with a single blow the vital force theory. By synthesizing urea from inorganic compounds, he demonstrated that something found and produced by a living organism could be obtained in the laboratory from a non-living chemical compound, ammonium cyanate. There was no need for a mysterious vital force supposedly present in living plants and animals to generate organic compounds. Both living and non-living matter obeyed to the same rules: those of chemistry. In Wöhler words, what he achieved was "the great tragedy of science, the slaying of a beautiful hypothesis by an ugly fact." And facts are what scientists believe in.

Urea was first isolated from human urine by a French chemist, Hillaire Rouelle, in 1773. Our body (and those of other mammals) produces urea from the metabolism of proteins, with a process called Urea cycle, elucidated only 150 years later by Krebs and Henseleit. In mammals, this process is performed in the liver, which breaks down proteins (obtained from a nice steak, for example) into its components, the amino acids. Some of them are recycled as-is, to build more proteins for our body, while the excess ones are eliminated. Amino acids contains nitrogen atoms, and an efficient strategy to purge them has been evolved by relocating them into the formation of urea, a strategy with many advantages: urea is inert, very soluble and contains a lot of nitrogen, making it an ideal and efficient carrier for translocation and elimination.

Another issue troubling chemists contemporary to Wöhler, and Wöhler himself, was that substances with the same constituent atoms could have different properties. Bear in mind that we are talking of the early days of laboratory chemistry, and chemical formulas were evaluated by burning stuff and measuring the obtained products: CO2 from carbons, water from hydrogens. Nitrogen could be determined by converting it to ammonia, and oxygen was assumed to fill up the missing difference. As you can imagine, it was error-prone at best. Nevertheless, accurate molecular formulas could be obtained, and first instances of completely different compounds with the same molecular formula started to appear, cases of what we know today as isomerism: different arrangements of the same atoms bring different properties. Take ethanol and ether as an example: you can drink the first and have a good time, but the second is going to be a very, very bad experience. Both have the same molecular formula: C2H6O. Synthesis of urea from ammonium cyanate The Wohler synthesis was another instance of a transformation involving isomerism. Both had the same molecular formula, but different properties. In this sense, Wohler actually achieved two feats in one shot: disproved vitalism, and provided rock solid evidence for isomerism.

image

As you can see from these findings, Wöhler was a remarkable and proficient chemist. In addition to urea synthesis, he is also known for isolating, discovering or characterizing both natural compounds (benzaldehyde, uric acid) and inorganic metals (aluminium, beryllium, yttrium, silicon, nickel, cobalt). He had a very interesting and polihedric personality: calm and patient, focused on facts rather than speculation, avid collector of meteorites and roman anquities, interested in chemistry, mathematics, arts, history.

Today, Urea is produced from ammonia (NH3) and carbon dioxide (CO2) in a high pressure, high temperature reactor. This mixture produces ammonium carbamate (NH2COONH4). Removing a molecule of water from this salt produces urea, which is then "prilled" (solidified into granules) by spraying it from the top of a very high tower (the so-called "prilling tower"). Urea is incredibly central in our modern life: it is an optimal fertilizer, due to its high concentration of nitrogen. It slowly releases two ammonia molecules and CO2, allowing a gradual release of nitrogen which is then fixed by soil bacteria and made available to the plant. With malonic acid, it forms barbituric acid, a base for pharmaceutical compounds. Production of melamine/urea-formaldehyde resins allows production of glues and other plastics.

Wöhler findings were just the beginning of a series of discoveries and realizations in organic chemistry, whose legacy is still pervasive today. It stimulated major advancements in biochemistry, inorganic chemistry, spectroscopy, theoretical chemistry, and analytical chemistry.

Links