| Author: | Stefano |
|---|---|
| category: | Chemistry |
| tags: | benzene, molecules |
Except for its nice regular hexagonal shape, benzene is not a nice compound. It is toxic, carcinogen, highly flammable, burns with a very dirty and smoky flame, and if it is not enough, it made chemists go crazy for one hundred years. The latter point is interesting for our discussion. Why, you may ask ? It has to do with its structure and a bunch of data that didn't add up for quite a long time, starting from its discovery, in 1825. It took more than one hundred years to finally understand what was going on, and it required the development of a whole new scientific discipline: quantum chemistry.
Our story begins when chemistry was a very smelly, messy and toxic affair (it still is, but we are more careful). At that time, there was no electron, the concept of bond just started to appear in the literature, even atomic weights were wrong: carbon was assumed to weight 6 daltons instead of the correct 12, hence all the formulas were computed to have twice as much carbon. No wonder nobody could figure anything out.
Benzene, also known as benzol, was first isolated by Michael Faraday by boiling crude oil and condensating the obtained vapors, a procedure known as distillation. The same compound was subsequently found in coal tar , and produced from some vegetal resins. Due to its characteristic smell, chemists declared benzene as the opener of the newly minted class of aromatic compounds.
When something new was isolated in the 19th century, the first thing chemists did was to burn it and gather the combustion products, to derive its composition. The process, known as combustion analysis was one of the very few tools available at the time to understand the internal nature of substances; it exploits the fact that atoms are not created nor destroyed, but just moved around to create new molecules (the so-called Lavoisier rule of conservation of mass). Benzene was clearly thrown into the ordeal, and after a bit of math, the resulting rate between carbon atoms vs. hydrogen atoms was found to be 1:1 (the empirical formula), that is, there was one hydrogen atom for each carbon atom. Additional experiments determined the molecular formula to be C6H6. A good start, but nothing was said about how the atoms were arranged together, the structural formula. Here the problems began to accumulate.
In stable, "normal" molecules, carbon connects with four other atoms via single bonds. Eventually, it can connect to the same atom with double or triple bonds, but the total number of bonds has to be four. This was acquired but still not settled knowledge when benzene was investigated. Chemists knew the double bond between two carbon atoms was quite reactive, and glad to open to become a single bond while adding something else to the remaining opened bond. This reaction is known as addition:
Benzene did not behave anything like that. It was assumed there were double bonds because the amount of hydrogen was very low, but it was incredibly inert, and did not perform the addition reaction. Instead, it performed an unusual reaction, substitutions. On top of this, more weird facts: when something was attached to it only one product was obtained, as if only one carbon was accepting the new appendage, or as if all carbons were equivalent. However, if two molecular groups were attached, three products were obtained! Gathering all this strange behavior, the problem now was to find a structure satisfying all the facts.
Trying to make sense to all these data, Freidrich Kekulé proposed in 1865 a hexagonal structure, where carbon atoms were connected via alternated double and single bonds, with the remaining bonds connecting to hydrogens.
The circular structure was not, however, whole Kekulé invention. Similar proposals were made by Joseph Loschmidt in 1861, but from his notes it is difficult to say if he really meant it. Archibald Couper had similar insights even earlier, in 1858, but a nervous breakdown arising from unfair treatment put him out of scientific activity.
Alternative structures were proposed by Claus (Claus benzene, which cannot exist) in 1867, Dewar (Dewar benzene, which can be produced, but it is highly unstable) in the same year, and Prismane, proposed by Ladenburg in 1869. The proposal made by Kekulé appeared the most likely to be correct. It explained the single product when something was added to it
as you can see, it does not matter where X is added. All carbons are equivalent, because if you substitute, say, on carbon 2, you can just rotate the molecule and make it overlap exactly with the one shown.
It also explained the three products when two groups were added
these three structures are different, and are therefore three totally different products. One objection, however, quickly arised: if you have X on carbons 1 and 2, and X on carbon 1 and 6, rotate the second molecule and try to make them overlap by renumbering the atoms, they are not the same compound. In the first case they embrace a double bond, and in the second case, a single bond
According to logic and what was known, these two molecules were different, thus not matching the "three products" fact. Kekulé solved this fallacy of his model by stating that the two structures quickly interconverted one into the other, so you could not see the difference. He was not that far from the truth, but he was wrong nevertheless.
Time passes, and we are now well into the 20th century. More evidence at the same time both confirmed and did not match the Kekulé model. For example, if you take a compound with a double bond, say, ethylene, and make it react with hydrogen (treating it very badly in a high temperature reactor)
CH2=CH2 + H2 -> CH3-CH3 + Energy
the double bond disappears and you get ethane and some energy. Let's call this amount of energy E. Repeat the same for a compound with two double bonds, and you get two times E. With three double bonds? You get three times E. This makes sense. What it didn't make sense was the fact that performing the same operation on benzene (which had, apparently, three double bonds) did not give three times X. It gave much less! It was like one double bond did not exist. The stable, inert and unusual behavior for a compound made of all those double bonds was baffling. At least it was found that forcefully adding hydrogen to benzene produced cyclohexane, confirming the fact that it was indeed circular.
Another problem arose when X-ray technologies allowed to measure the length of the bonds between atoms. In non-benzene molecules, Carbon-Carbon double bonds were found to be shorter (134 picometers) than single bonds (154 picometers). This would have produced a distorted hexagon, with short sides (for double bonds) and long sides (for single bonds). The measures performed on benzene were unambiguous: the carbon-carbon bonds were all the same length: 139 picometers, a measure in between a Carbon-Carbon double bond and a single bond.
It took Linus Pauling with the concept of resonance, and the work of many famous theoretical chemists such as Hückel (with the method and rule named after him) to explain what was going on: simplifying a lot, benzene does not have double bonds and single bonds. It has bonds that are all the same, and are a "mixture", an intermediate between a single and a double bond. Resonance is the phenomenon that allows mixing of two or more molecular structures (called resonance structures) to define a much better representation of reality: a resonance hybrid. Resonance displaced the concept of "rapid interconversion" of structures defined by Kekulé. The benzene bonds were not rearranging quickly from single to double and vice-versa. They were something in between!
We can make this strange phenomenon more clear with a (slightly crazy) comparison against a real-world situation: if you inbreed a horse and a donkey what you obtain is a mule, which is an animal in its own right and is a mixture of the two original animals. The mule can be seen as a resonance hybrid defined by the donkey and the horse resonance structures. Thinking in Kekulé terms, we would say the mule spends half of its existence as a horse, and the other half as a donkey, quickly transforming from one to the other, which is not the case.
The concept of resonance also explained the perfect hexagonal shape of benzene: each bond is not single nor double; it is instead a mixture whose length is intermediate between the two, and each bond has exactly the same length. It also explained the unusual stability of benzene: the electrons are not concentrated between specific atoms (which makes them more likely to engage in a reaction). Resonance spread them on a larger, uniform and diffuse ring, with the consequence of reduced reactivity. Be warned though, this is a strong simplification. A proper explanation would require some math.
The concept of resonance pushed organic, inorganic, theoretical and computational chemistry into a completely different realm of understanding. It explains with ease many apparently puzzling chemical reactions, as well as the properties of molecules in terms of stability, optical properties (for example, their color) and interaction with other molecules. It explained why the black stick you have inside your pencil (graphite, which is pure carbon) conducts electricity and is black, while the shiny, expensive rock you find on engagement rings (diamond, again pure carbon) is totally non-conductive and transparent. It allowed to further develop our understanding of molecular reactivity, so that we can now synthesize complex compounds such as graphene, fullerene, and nanotubes, all made from the hexagonal unit benzene, repeated again and again.
As for benzene itself, it is so central to our current lifestyle that living without it is hard to imagine. Benzene is in fact the starting point for the production of drugs, plastics, detergents, pesticides, paints, dyes, rubber, explosives, as an additive to fuel, and as a solvent. Considering that most benzene is produced from oil, we can see how a shortage of crude oil can have a major impact on many different aspects of our life.
- http://www.eco-usa.net/toxics/chemicals/benzene.shtml
- http://www.chemguide.co.uk/basicorg/bonding/benzene1.html
- http://www.chemguide.co.uk/basicorg/bonding/benzene2.html
- http://videos.howstuffworks.com/science-channel/27865-100-greatest-discoveries-august-kekules-written-model-video.htm
- http://realtalklibrary.com/2009/08/03/friedrich-august-kekule/
- http://www.statemaster.com/encyclopedia/Friedrich-August-Kekule
- http://stainsfile.info/StainsFile/theory/science/benzring.htm
- http://www.tutorvista.com/content/chemistry/chemistry-iii/hydrocarbons/benzene-structure.php
- http://www.bbc.co.uk/dna/h2g2/A38528111
- As derivative work, credits to the providers of Wikipedia images for the donkey, horse and mule.
