August Kekulé

Chapter 1 · The Architect Who Went to a Lecture

Giessen, 1847

1829 – 1852 · Darmstadt · Giessen · Paris · London

Friedrich August Kekulé was born at Darmstadt in September 1829, into a civil-service family. He was good at drawing and at languages, and went to the University of Giessen in 1847 intending to become an architect.

He attended Justus von Liebig's lectures, and changed course permanently.

The architectural training did not go away, and it is the most useful thing to know about him. An architect thinks about how components join, what a structure can support, and what arrangements are possible given the constraints of the parts. Kekulé brought exactly that habit to molecules — and where other chemists asked what a compound *contains*, he asked how it is *built*.

He took his doctorate at Giessen in 1852, then went to Paris, where he met Charles Gerhardt and Charles-Adolphe Wurtz, and absorbed the French type theory of organic compounds.

He spent 1854 to 1855 in London, working as an assistant and — importantly — talking a great deal with Alexander Williamson, who had shown that ether is formed by one alkyl group replacing a hydrogen in an alcohol, and who thought in terms of atoms in arrangements.

Kekulé later said that on a London omnibus one summer evening he fell into a reverie and saw atoms dancing before his eyes, the smaller ones joining in pairs, larger ones taking hold of two or three of the smaller — and that he saw how they linked in chains.

That is the *first* of his two dream stories, and like the second it was told long afterwards.

“Let us learn to dream, gentlemen, then perhaps we shall find the truth — but let us beware of publishing our dreams before they have been put to the proof by the waking understanding.”

— August Kekulé, Benzolfest address, Berlin, 1890

Why this matters

Kekulé's architectural training gave him the habit of asking how a molecule is built rather than what it contains, which is the question organic chemistry needed.

You have the architect and the omnibus. What would you ask him?

Ask Kekulé

• “What did architecture teach you that chemistry did not?”
• “What did Liebig's lecture actually do to you?”
• “What did you learn from Williamson in London?”
• “Tell me about the omnibus.”
• “Why does it matter how a molecule is built rather than what is in it?”

Chapter 2 · Four Bonds, and Carbon to Carbon

1857 and 1858

1857 – 1858 · Heidelberg · Ghent

Two claims, published a year apart, made organic chemistry possible.

The first, in 1857: *carbon is tetravalent*. In every compound it forms, a carbon atom makes exactly four bonds. Not sometimes three, not sometimes five. Four, always.

The second, in 1858: *carbon atoms bond to one another*. A carbon can spend one or more of its four bonds on another carbon, so carbons can be strung into chains of any length, with the remaining bonds taken up by hydrogen or other atoms.

Together they explain three things at once.

Why there are so many carbon compounds. If carbon can link to itself indefinitely, the number of possible structures is effectively unlimited — which is why organic compounds outnumber all others by orders of magnitude.

Why compounds come in *families* differing by one carbon at a time. The alkanes — methane, ethane, propane, butane — are simply chains of increasing length, and their properties change gradually along the series.

And why *isomers* exist. Four carbons can be joined in a straight chain or with a branch, giving two different substances with the same formula. That is the answer to the puzzle Wöhler and Liebig had found thirty years earlier.

Archibald Scott Couper, a Scottish chemist working in Wurtz's laboratory in Paris, reached the same two conclusions independently and at the same time — and, in one respect, went further, drawing lines between atoms to represent bonds, which is closer to modern notation than Kekulé's own sausage-shaped diagrams.

Couper gave his paper to Wurtz to present to the Academy. Wurtz delayed. Kekulé's paper appeared first. Couper protested angrily, was expelled from the laboratory, suffered a breakdown from which he never fully recovered, and did no further scientific work. He was twenty-seven.

Why this matters

Tetravalent carbon that bonds to itself explains at one stroke why organic compounds are so numerous, why they form families, and why isomers exist.

You have the two claims and the man who lost the race. What is your question?

Ask Kekulé

• “Why does carbon always make four bonds?”
• “What follows from carbon bonding to carbon?”
• “How does this explain isomers?”
• “What happened to Archibald Scott Couper?”
• “Couper's notation was better than yours — do you concede that?”

Chapter 3 · Presiding at Karlsruhe

1860, and the end of fifty years of confusion

1860 · Karlsruhe · Ghent

Kekulé's structural theory had a problem. It depended on knowing how many atoms of each element a molecule contains — and in 1858 chemists could not agree on that, because they could not agree on atomic weights.

Without Avogadro's hypothesis there was no reliable way to get from mass measurements to molecular formulae, and different chemists used incompatible systems. Kekulé wrote that chemists could not understand one another.

His structures were useless if nobody agreed what C₄H₁₀ meant.

So in 1860 he, Charles-Adolphe Wurtz and Karl Weltzien organised an international congress at Karlsruhe — the first meeting of its kind — for the express purpose of settling the question. About 140 chemists came from across Europe.

Kekulé opened the proceedings.

The congress did not vote a resolution and did not formally settle anything. What happened instead was that Stanislao Cannizzaro spoke, explaining how Avogadro's fifty-year-old hypothesis gives a consistent method for determining atomic weights, and had copies of his pamphlet handed out as delegates left.

It worked by conversion rather than by decree. Lothar Meyer read the pamphlet on the journey home and said the scales fell from his eyes. Mendeleev was in the room and produced the periodic table nine years later.

Kekulé's own role was organisational rather than intellectual, and it was essential. Somebody had to decide that the discipline needed to sit down in a room and sort it out, and then arrange for it to happen.

After Karlsruhe, his structures had firm numbers to work with.

Why this matters

Karlsruhe settled nothing by vote and everything by persuasion, and it is the founding instance of a scientific discipline convening to agree its own conventions.

You have the congress and the pamphlet handed out at the door. What would you ask?

Ask Kekulé

• “Why did your structures need the congress to happen?”
• “What did it take to get 140 chemists into one room?”
• “Why did persuasion work where a vote would not?”
• “What was it like when Cannizzaro spoke?”
• “Should sciences meet like this more often?”

Chapter 4 · The Ring

Benzene, 1865, and a structure no chain could give

1865 · Ghent

Benzene was a serious problem. Faraday had isolated it in 1825 and its formula was known: C₆H₆.

Six carbons and only six hydrogens. On Kekulé's own theory, six carbons in a chain should carry fourteen hydrogens. Benzene has *eight fewer* than it should. It is enormously *unsaturated* — and yet it does not behave like an unsaturated compound at all. Alkenes react readily by addition; benzene is remarkably unreactive, and when it does react it *substitutes* rather than adds.

And there is another clue. When one hydrogen is replaced, only *one* monosubstituted product exists — all six hydrogens are equivalent. When two are replaced, exactly *three* isomers exist, never more.

In 1865 Kekulé proposed a *closed ring* of six carbons, each bonded to one hydrogen, with alternating single and double bonds around the ring.

A ring makes all six positions equivalent, giving one monosubstituted product. It accounts for the missing hydrogens. And it predicts the right number of disubstituted isomers — though the alternating bonds should give *four*, and only three exist, which Kekulé patched in 1872 by proposing that the double bonds oscillate rapidly between two arrangements.

That patch was a genuine problem and he knew it. The real answer — that the electrons are *delocalised* around the ring, belonging to no particular pair — required quantum mechanics and arrived in the 1930s.

In 1890, at a jubilee held in Berlin to mark twenty-five years since the ring, Kekulé told an audience that the structure had come to him in a reverie before the fire, in which he saw chains of atoms twisting like snakes, and one of them seized its own tail.

The story is told twenty-five years after the event, by a man being honoured, in a speech. Historians have doubted it ever since — and it is worth noting that in the same speech he warned against publishing dreams before testing them awake.

Why this matters

The benzene ring explains why all six hydrogens are equivalent and why the compound substitutes rather than adds — and Kekulé's oscillating bonds were a patch that waited seventy years for delocalisation.

You have the ring, the missing isomer and the snake. What is your question?

Ask Kekulé

• “What is wrong with a chain of six carbons for benzene?”
• “How does a ring explain that all six hydrogens are the same?”
• “Your alternating bonds predict four isomers and only three exist — what then?”
• “Did the snake dream actually happen?”
• “Why warn against publishing dreams in the same speech?”

Chapter 5 · Bonn

Four students, four Nobel Prizes, and a death five years too early

1867 – 1896 and after · Bonn

Kekulé took the chair at Bonn in 1867 and stayed until his death.

His private life was hard. His first wife, Stéphanie Drory, whom he married in 1862, died in childbirth in 1863 after eighteen months of marriage. He was devastated, and colleagues thought his work never recovered its earlier intensity. He married again in 1876, to Luise Högel, his housekeeper — a match that scandalised parts of Bonn society.

His laboratory at Bonn trained an extraordinary set of chemists. Jacobus van 't Hoff, who in 1874 proposed that carbon's four bonds point toward the corners of a tetrahedron — the three-dimensional step Kekulé himself had not taken — and who won the first Nobel Prize in Chemistry in 1901. Emil Fischer, who worked out the structures of the sugars and proposed the lock-and-key model of enzymes, Nobel 1902. Adolf von Baeyer, who synthesised indigo, Nobel 1905.

Of the first five Nobel Prizes in Chemistry, three went to men who had worked under Kekulé. He died in July 1896, five years before the first was awarded.

He was ennobled in 1895, becoming Kekulé von Stradonitz.

His structural formulae are the permanent legacy, and they are odd things when you look at them. A drawing of a molecule is not a picture — atoms have no lines between them — and yet a chemist reasons with those drawings constantly, predicting reactions from them. Kekulé invented a *notation that supports thinking*, which is rarer and more valuable than a notation that merely records.

Every structural formula drawn anywhere is his invention still in use.

Why this matters

Three of the first five Nobel Prizes in Chemistry went to men trained in Kekulé's laboratory, and he died five years before the first was awarded.

You have Bonn, the students and the drawings. What would you ask him?

Ask Kekulé

• “Why is a structural formula not a picture?”
• “Why did you not take carbon into three dimensions?”
• “What did Stéphanie's death do to your work?”
• “What did you teach that produced three Nobel laureates?”
• “Can a drawing be an argument?”

What Kekulé changed

Every structural formula a student draws is Kekulé's invention in use. The tetravalency of carbon and its ability to bond to itself are the first two facts of any organic chemistry course, and the benzene ring remains one of the most recognisable diagrams in science. He also organised the Karlsruhe congress that finally settled chemistry's atomic weights.

A debate that continues

Archibald Scott Couper reached tetravalent carbon and carbon chains independently and was ruined by the delay in publishing; and the snake-dream account of the benzene ring was told twenty-five years after the fact and is widely doubted.

Keep exploring — ask Kekulé

• “What should Couper be remembered for?”
• “How much does a good diagram do for a science?”
• “What would you have drawn with the electron in hand?”

Related lives

• Friedrich Wöhler — Who Made Urea Without A Kidney
• Justus von Liebig — Who Weighed What Burned Away
• Emil Fischer — Who Said A Lock Must Fit Its Key
• Amedeo Avogadro — Whose Idea Waited Fifty Years

Related themes

Organic structures · Bonding in carbon compounds · Isomerism