August Kekulé

A draftsman who fell in love with molecules

Friedrich August Kekulé von Stradonitz was born on the 7th of September, 1829, in Darmstadt, the small Hessian capital where his father held a post in the war ministry. He grew up in a comfortable house full of books and was, by every account, a talented draftsman as a boy. He could sketch a building from memory after one walk past it. When he enrolled at the University of Giessen in 1847 he was not headed for chemistry at all. He went to study architecture, intent on a career of stone and plan-rooms, and for two years that is exactly what he did.

What changed him was a murder trial. The Countess von Görlitz was found dead in her locked room in 1847, and the great chemist Justus von Liebig was called in as an expert witness for the prosecution. Kekulé attended the public lectures Liebig gave on the case and on chemistry generally, and the encounter undid him. He abandoned architecture, switched faculties, and became one of Liebig’s students. The draftsman’s habit of seeing buildings in three dimensions stayed with him for the rest of his life. He had simply changed the subject from houses to molecules.

Paris, London, and a vision on the Clapham omnibus

After his doctorate at Giessen in 1852, Kekulé spent a few years wandering the laboratories of Europe. He worked under Adolphe Wurtz in Paris, alongside Charles Gerhardt and Auguste Laurent, where he absorbed the French school’s growing conviction that molecules had a definite internal architecture. Then in 1854 he moved to London for a stretch as assistant to John Stenhouse at St. Bartholomew’s Hospital. He was twenty-five, broke, and surrounded by the smoke of an industrial capital that ran on coal-tar chemistry.

It was on a London bus, by his own later account, that he first saw atoms dancing. He had been sitting up top one summer evening in 1855, half-dozing, when he watched the streetlamps and the carriages and imagined molecules linking and unlinking in the air around him. The image gave him the first hint of what he would later call the tetravalent nature of carbon: each carbon atom forming bonds with exactly four neighbours, the four hands of a chemical glove. He published the idea in 1858 in Liebig’s Annalen, the same year an unrelated Scotsman, Archibald Scott Couper, arrived at the same insight independently. Couper’s paper was delayed in Paris and Kekulé got the credit. Couper had a breakdown and never published again. Chemistry is not always kind.

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The Bonn chair, and a snake that bit its tail

In 1858 Kekulé took the chair of chemistry at Ghent, in newly independent Belgium, and in 1867 he was called to Bonn, where he would remain for the rest of his career. He built up one of the great teaching laboratories of nineteenth-century Europe. Three of the first five Nobel Prizes in Chemistry went to his former students: Jacobus van ‘t Hoff, Emil Fischer, and Adolf von Baeyer. A fourth, Hermann Emil Fischer’s own student, went to Otto Wallach. Kekulé did not live to see the prizes (they began in 1901, five years after his death) but the chain of chemists who learned at his bench shaped half the discipline.

The problem he was best known for solving was benzene. C₆H₆ had been isolated by Michael Faraday in 1825 from the oily residue of London gaslights, and for forty years no one could explain its formula. Six carbons and only six hydrogens meant each carbon was missing three hands. The valence accounting did not work. In 1865 Kekulé published a paper in the Bulletin de la Société Chimique de Paris proposing that the six carbons formed a closed ring with alternating single and double bonds, each carbon bonded to one hydrogen. The hands balanced. The whole strange family of aromatic compounds (toluene, phenol, aniline, the dyestuffs of an emerging German industry) suddenly made sense.

Years later, at a banquet held in 1890 to mark the silver anniversary of the benzene paper, Kekulé told the story of how the idea had come to him. He had been dozing in front of a fire in Ghent, he said, and saw atoms whirling in long chains, “twisting and turning in a snakelike motion.” Then one of the snakes seized its own tail, “and the form whirled mockingly before my eyes.” He woke and spent the rest of the night working out the consequences. The image (the ouroboros, the snake biting its tail) was probably embellished for a banquet audience. Modern historians are divided on how much of the dream actually happened and how much Kekulé arranged in retrospect. But the ring was real, the bonds were drawn, and the next generation of organic chemistry was off and running on six-membered rings.

What he means to the quantum story

Kekulé died on the 13th of July, 1896, at sixty-six, just over a year before J. J. Thomson would announce the electron and force chemists to ask what a “bond” actually was. Kekulé had drawn the bonds without knowing. He had given chemists the picture (carbon with four hands, a ring of carbons holding hands in a hexagon) that quantum mechanics would later have to explain. When Linus Pauling worked out the resonance structure of benzene in the 1930s, and when Erich Hückel computed the delocalised π-electrons that actually hold the ring together, they were giving a quantum answer to a question Kekulé had framed in pencil and paper. The architecture student from Darmstadt taught chemistry to see in three dimensions. The quantum theorists had to learn to draw what was already there.