Tuesday 23 April 2024

Benzene

The chemical benzene is apparently an attractive proposition for a number of artists.
Benzene has a unique spatial structure that makes it stand out from other molecules. It has been known since the 1800's that benzene consists of six carbon atoms and six hydrogen atoms. It was also known that carbon-based molecules are generally spatially arranged in connecting tetrahedrons. As this is impossible to achieve with an equal number of carbon and hydrogen atoms, it has been a long standing mystery on how these atoms were arranged in the molecule.
The beginning of the solution was offered in 1865 by August Kekulé, who proposed a geometrically flat hexagonal 'ring' structure with alternating 'double bonds', which he visualised in the following manner:


This is the actual model Kekulé built to demonstrate the structure. It is now in the collection of the University Museum in Ghent, Belgium, where Kekulé was living at the time.
In this model the black balls represent carbon atoms, the white balls are hydrogen atoms and the connecting metal rods are single and double bonds. Such bonds are connections between atoms created through both atoms 'sharing' a pair of electrons. In a double bond two pairs of electrons are thus shared between two adjacent atoms.
After the 19th century, quantum mechanics and molecular orbital theory have further refined this view. Yet the general principle of benzene consisting of six carbon atoms in a planar hexagonal shape still stands, with the six hydrogen atoms arranged like 'antennas' at opposite ends of, and in the same plane as, the hexagon. As such, it is usually graphically depicted in one of the following ways:

It's obvious that artist Monira Al Qadiri had these (simplified) graphical structures in mind when she started on her work 'Benzene' in 2022. This work consists of a series of sculptures where, according to her, 'the scientific geometry of benzene's chemical compounds are rendered into glass sculptures, in order to highlight the grip that this perfumed molecule has on our lives.' While most of these structures are straightforward translations of the above shown schematics into three-dimensional glass shapes, one of them caught my attention:

To any chemist it's immediately obvious that this is completely impossible and has nothing to do with any kind of reality. Al Qadiri's claim to 'the scientific geometry' is thus hardly scientific.
To understand why this is so you need a bit of technical understanding about delocalized π-electrons in the structure of benzene and where possible lone pairs of electrons would go if they hydrogen atoms were displaced. Since providing such understanding isn't really attainable within the scope of this blogpost, let me just say that Al Qadiri's sculpture is a bit like stating that this is what a functional bicycle looks like:

Al Qadiri further places emphasis on the smell of benzene. She says that benzene is 'a colourless and highly flammable liquid with a sweet smell, it is partially responsible for the aroma around petrol stations, and is thus classified as an ‘aromatic hydrocarbon.’ ' It is in this manner that she makes the connection between benzene and the petrochemical industry. While benzene is (non-exclusively) extracted from crude oil, the connection she makes with petrol stations is partially a false one. Benzene is a minor part of gasoline, of only approximately 1% by volume. It thus doesn't contribute greatly to any particular core property of gasoline, least of all it's flammability. This flammability is much more influenced by short-chain alkanes like butane and hexane, which have far lower boiling points and oxidize much more rapidly. In fact, a mixture of benzene and benzene-like molecules called BTEX is sometimes added to gasoline to reduce its combustibility.
The second part of her statement, where she links the smell of benzene to its classification as an 'aromatic hydrocarbon' misunderstands cause and effect. It is true that in 1855 August Wilhelm Hoffman gave the classification of 'aromatic acids' to a number of compounds, even if not all them had  a distinctive smell. We now know the core component of those 'aromatic acids' was the presence of a benzene-like structure and the 'aromatic' moniker has thus stuck for those kind of molecules. Their properties and uses vary wildly, however. Besides benzene, photographic developer is also aromatic and so are the basic building blocks of DNA. Al Qadiri's observation is thus far removed from an explanation of any of benzene's properties or 'the grip it has on our lives'.

Less poetically interpretative, but equally misinformed, is a much earlier example by the hand of Bernar Venet. At the time known for his appropriated 'scientific' drawings, Venet made the following 'drawing' in 1966:

The text, in French, describes the 'importance of Kekulé's formula'. According to Venet this 'formula allowed us to interpret the hydrogenation of benzene to cyclohexane and the chlorination to benzene hexachloride.'
I'm not exactly sure what he's attempting to express here. Hydrogenation of alkenes to alkanes using platinum catalysts was first published in 1874, some ten years after Kekulé's discovery, and I'm not entirely sure this process would work on the more stable structure of benzene. How it helps us 'interpret' this process is thus unclear to me, as it implies that the discovery of the process came after the explanation that process.
Furthermore, the process of 'chlorination' generally refers to simple addition of chlorine to a molecule. In this case you would thus end up with hexachlorocylcohexane, not hexachlorobenzene as is claimed in Venet's text. But it is possible to make hexachlorobenzene from benzene with a substitution reaction, so lets just assume Venet meant this instead. In that case he describes the structural formula of 'hexachlorobenzene' as C6H12Cl6. This formula is simply impossible. A carbon atom can only be connected to four other atoms at the same time. In a ring structure, two of those possibilities are already taken up by the neighbouring carbon atoms, which leaves us with a total of 12 'free' spaces. As we have 6 chlorine atoms and 12 hydrogen atoms in Venet's proposed formula, we apparently need to fit 18 atoms into the 12 available possibilities. The correct formula would thus be C6H6Cl6 for hexachlorocyclohexane or C6Cl6 for hexachlorobenzene.
Furthermore, in order to show the formation of benzene, Venet uses the so-called trimerisation of alkyne as his example. This is an unusual choice from a chemical point of view. Firstly because this process was first described in 1866, one year after Kekulé published his formula. And secondly because t
rimerisation is a very difficult reaction to perform. It has a very high activation energy, thus requiring high temperatures of >800 ºC, and even then the end result isn't pure benzene but a mixture of different products. Therefore this reaction was far from efficient, or common, until a different process was developed in the late 1940's that involves the use of catalysts, which made alkyne trimerisation a viable reaction in routine synthesis work.
Thus while I generally enjoy the drawings of Venet for their stylized simplicity, it's best to not actually read the text that's contained in them.

Richard Venlet is a third artist I've encountered who has an interest in benzene and it's structure. He published a booklet with the title Kekulé in 2011. Its starting point was the anecdote of August Kekulé's first insight into the structure, which took place while he was living in Ghent, Belgium. 

Venlet presents no claims to scientific knowledge and his little booklet seems to be nothing more than a happenstance that reflects his interest in hexagonal shapes, like the ones he used for a series of floor panels created for Maniera two years prior:

As is clear to see, in the booklet Venlet presents a repeating pattern of hexagons, as he would show in his exhibitions. The reference to Kekule and his structure of benzene is thus a pure formal one. Nevertheless, it must be pointed out that the structure he presents is chemically impossible. The flat structure of such a system, like in the well-known graphene, is only possible through the 'double bonds' present in benzene, or rather its delocalized π-system. Such a system is usually represented by the addition of extra lines in certain places, which are missing in Venlet's drawing. Although seemingly a small difference, this has great consequences for the spatial arrangement of such a molecule, which would put an impossibly large strain on the system that Venlet represents. The following illustration hopefully gives a sense of the factual differences that are left out of such simplified illustrations:

It should be easy to see that a so-called saturated system that Venlet has drawn is far more crowded and therefore possesses very little room to move and wiggle, something all atoms want to do. While it might be possible on a smaller scale like the above illustration, a large field like the one presented in Venlet's booklet will in reality simply fall apart and find a different conformation.

In conclusion I should once again state that although I have never expected otherwise and can occasionally enjoy the fantasy-rich interpretations of artists, it's nevertheless a good idea to presume that an artist's factual understanding of the natural sciences is negligible. When I asked as a chemist I know why he enjoyed working with artists, he simply said 'it's so nice to see people who are unburdened by knowledge'.