The discovery that the molecule benzene is built around a ring of carbon atoms joined to one another as if holding hands opened up a new branch of chemistry that would eventually provide key insights into the nature of the molecules of life. Credit for the image of the benzene ring goes to the German August Kekulé, who published the idea in 1865. But the experimental work on which the discovery was based had largely been carried out by the Scot Archibald Scott Couper, working in Paris in the 1850s, and Joseph Loschmidt, a German based in Vienna, a little later. Although Kekulé did his own experiments, these were similar to those of the other researchers, and, as Couper came first, we will describe his approach here.
The compounds that were important in this work were known as ‘aromatics’, because of their pleasant smell. They could be derived from benzene and their molecules are, we now know, built around the benzene ring. As a result, all compounds containing a benzene ring, or indeed other flat rings, are now called aromatics, whatever they smell like.
Benzene has a very unusual chemical formula, C6H6, meaning each molecule is made up of six carbon atoms and six hydrogen atoms linked in some way. Just how they were linked was a mystery prior to 1865, but as a step towards working out the structure Couper experimented with ways to convert benzene into related compounds containing the hydroxyl group (or radical), OH. These compounds include C6H7OH and C6H6(OH)2. He also studied salicylic acid, C6H4(OH)COOH, and tried to find theoretical structures that could explain the properties of these compounds. In doing so, he was the first person to realize that carbon atoms could link to one another to form chains, and the first to attempt to depict the physical arrangement of carbon atoms within the benzene molecule. Early in 1858, Couper wrote a paper on his work and gave it to the head of his laboratory, Charles Adolphe Wurtz, to pass on to the French Academy of Sciences. Because Wurtz sat on the paper for several weeks, it was presented to the Academy only in June, a month after Kekulé published a paper announcing similar results. So Kekulé got the credit that Couper thought he deserved. This led to a furious row with Wurtz, who threw him out of the laboratory; Couper never developed the ideas further, leaving the way open for Kekulé.
By that time, the idea that atoms have a certain ‘valence’ had been established. This is a measure of the ability of an atom to make chemical bonds with other atoms. Hydrogen has a valence of one, so can form one bond; oxygen has a valence of two, so can form two bonds. In a molecule of water, H2O, one oxygen atom is linked to two hydrogen atoms, one for each bond, represented as O-H-O. Carbon has a valence of four, so it is easy to understand the structure of methane, CH4. But a key feature of Couper’s work was his willingness to consider the possibility that carbon atoms sometimes behave as if they have a valence of two. This occurs when a double bond links carbon to other atoms, as in carbon dioxide, CO2, which can be represented as O=C=O. Kekulé was against the idea at first, but it became a key feature of his revelation about the benzene ring.
‘Revelation’ is the right word, because, according to Kekulé, the solution to the puzzle came to him in a dream. In 1861, Loschmidt published drawings of suggested structures for several molecules, including some aromatic compounds. But in the case of the aromatics, he drew a circle where the benzene itself should be, to represent the unknown structure of the molecule. He did not, as has sometimes wrongly been claimed, suggest that the structure really was circular. It may be, though, that these drawings influenced Kekulé subconsciously. Kekulé published his idea of the benzene molecule as a ring in 1865; much later, he said that the idea came to him in day-dream in which he pictured a snake biting its own tail, like the mythical worm Ouroboros. The idea works because in a ring of six carbon atoms each atom can be joined to the neighbour on one side by a single bond and to the neighbour on the other side by a double bond, leaving the fourth bond free to link up with an atom of hydrogen or with any other atom, or to link to other rings. The importance of this discovery, which opened up a new area of chemical research, is highlighted by the fact that the molecules of life, DNA and RNA, are built around aromatic rings (see here).