In 1890 the German Chemical Society held a celebration in Berlin to mark twenty five years since August Kekule had proposed the structure of benzene. In his speech he told the audience how the idea had arrived. He had been dozing by the fire in Ghent, watching atoms dance in front of his eyes, when the chains of atoms twisted like snakes, and one of the snakes seized hold of its own tail and whirled mockingly before him. He woke, he said, as though struck by lightning, and spent the rest of the night working out the consequence.
It is the most famous anecdote in the history of chemistry. It is also, quite possibly, a story constructed for the occasion, and the reasons for suspecting that are more interesting than the anecdote itself.
Why Benzene Was a Problem
Michael Faraday isolated benzene in 1825 from the oily residue in gas lighting cylinders. By the 1860s its formula was known to be C6H6, and that formula was an embarrassment.
Carbon forms four bonds and hydrogen forms one. Six carbons in a chain, fully saturated, should carry fourteen hydrogens. Benzene has six. That degree of shortfall means a great many double or triple bonds, and compounds with those bonds are reactive: they take up bromine greedily, they are oxidised easily, they are chemically restless.
Benzene is not. It is unusually stable, it refuses to add bromine under normal conditions, and instead of adding it substitutes, swapping a hydrogen for something else and keeping its structure intact. Nothing in the chain based chemistry of the period accounted for a molecule that was formally starved of hydrogen and behaved as though it were content.
What Kekule Had Already Established
He was not an outsider guessing. In 1857 and 1858 Kekule had published the two principles the whole field then rested on: that carbon is tetravalent, forming four bonds, and that carbon atoms can bond to each other to form chains. The Scottish chemist Archibald Couper reached the same conclusions independently and slightly earlier, and his publication was delayed by his supervisor, which ruined his career and eventually his health.
Those two ideas turn organic chemistry from a catalogue into a structural science. If you know how many bonds each atom has and that they can link up, molecules acquire architecture.
Benzene was the case that would not fit the architecture, and Kekule’s answer, published in Paris in January 1865 and in German the following year, was to close the chain into a hexagonal ring of six carbons, each carrying one hydrogen, with alternating single and double bonds around it.
The Fix He Had to Add
The ring solved the count and left a visible problem. With alternating bonds, two carbons joined by a double bond are not equivalent to two joined by a single bond, so a molecule with two substituents on adjacent carbons should exist in two distinguishable forms.
Nobody could find the second form. In 1872 Kekule proposed that the molecule oscillates rapidly between the two arrangements, so that the two forms are averaged out and never separable.
This is a fudge in the sense that it was introduced to save the model, and it is also very nearly right. The modern account, which required quantum mechanics, is that the electrons in question are not in fixed bonds at all but delocalised around the ring, giving six identical bonds intermediate between single and double and an unusual stability that explains the chemical calm. Kekule’s oscillation is the closest thing to that picture available to someone working in 1872.
The definitive proof came in 1929, when Kathleen Lonsdale used X ray crystallography on hexamethylbenzene and showed the ring to be flat, regular and hexagonal, with all six carbon to carbon distances equal. She did the work at the Royal Institution, largely at home, with two young children, and it settled a sixty year old argument in a single paper.
The Case Against the Snake
The dream story appears in print for the first time in 1890, twenty five years after the discovery, in a speech to a room full of admirers. Kekule had never mentioned it in any paper, letter or lecture in the interval, which is a long silence for an anecdote that good.
The chemist John Wotiz spent years arguing that the story was a convenient construction, and the surrounding circumstances give it some weight.
- Priority was contested. Josef Loschmidt, a Viennese schoolteacher, had privately published a booklet in 1861 containing over 350 structural formulae, including representations of benzene and its derivatives drawn as closed shapes. Kekule owned a copy and referred to Loschmidt’s diagrams dismissively in print
- A dream cannot be contested. A vision by a fireside has no citations, no precursors and no debts. It removes the question of who read whose pamphlet
- The ouroboros was not an obscure image. A snake devouring its own tail was a standard alchemical and hermetic symbol, familiar to any educated German of the period
Defenders point out that Kekule described two separate dreams in that speech, the other on a London omnibus in 1855 giving him the idea of chains, and that the substance of both matches the sequence of his published work. Others note that scientists do report insights arriving in half sleep, and that there is no particular reason a chemist should be lying about it.
The honest position is that the structure is certainly his, in the sense that he stated it, defended it, and extracted testable consequences from it, and that the origin story is unverifiable and arrived suspiciously late.
What the Ring Was Worth
The practical consequence was enormous and immediate. Once benzene had a definite shape, the positions on the ring could be numbered, and a chemist could plan a substitution rather than discovering it by accident.
That is the foundation of the synthetic dye industry, which was the first science based industry in the modern sense. William Perkin had stumbled on mauve in 1856 while trying to make quinine, but stumbling does not scale. Structural theory turned dye making into design, and the German firms built on it, BASF, Bayer and Hoechst among them, became the largest chemical companies in the world within a generation.
Aspirin, sulfa drugs, explosives, plastics and most modern pharmaceuticals descend from ring chemistry. Somewhere between half and two thirds of all known organic compounds contain an aromatic ring.
Benzene itself turned out to cause leukaemia and is now handled with considerable care, which is a common ending for nineteenth century industrial chemistry.
Where It Sits
The line from Dalton’s atoms as counted weights, through Lavoisier’s insistence on weighing everything, to a hexagonal drawing on a page is a single argument getting more specific. First that matter comes in fixed proportions. Then that those proportions imply particles. Then that the particles have definite valencies. Then that they occupy definite positions relative to each other.
Benzene was the case that forced the last step, because it could not be accommodated by any arrangement in a line. It is also, together with Mendeleev’s table, one of the two nineteenth century structures that a modern chemist would still recognise as their own tool rather than as history.
The snake is probably a story. The hexagon is not.
No products in the cart. 