Richard Willstätter made a molecule that tested a theory

Person: Richard Willstätter
Lived: 1872–1942
Field: Organic chemistry
From: German

The story

Benzene, a ring of six carbon atoms, is unusually stable, and chemists of the early twentieth century wanted to know why. One obvious question was whether a larger ring built in a similar way, with alternating single and double bonds, would share benzene's special properties. Richard Willstätter, whose studies of the tropine alkaloids had given him great skill with ring-shaped molecules, took on the challenge of making such a compound.

The project took years, and accounts place its completion at the end of his time in Munich or during his years at the ETH in Zurich, which began in 1905. Starting from the alkaloid pseudopelletierine, he used a series of reactions known as Hofmann eliminations to build cyclooctatetraene, a ring of eight carbon atoms with alternating bonds that looks on paper like an enlarged benzene. His Nobel biography records it as the first synthesis of the molecule.

When he studied it, he found that it did not behave like an aromatic compound at all. It reacted much more like an ordinary chain of carbon atoms with double bonds, undergoing reactions that benzene resists. The result showed that benzene's stability could not be explained simply by its pattern of bonds. Willstätter also set up experiments to make cyclobutadiene, a ring of four carbon atoms, to test the same ideas.

His result was later doubted. Between 1939 and 1943 chemists in the United States failed to repeat the synthesis and suggested that he had made a different compound, styrene. During the Second World War Walter Reppe at the German chemical firm BASF found a simple way to make cyclooctatetraene from acetylene, and the product matched Willstätter's. In 1947 Arthur Cope and his co-workers at MIT repeated Willstätter's synthesis step by step, using his original techniques, and confirmed that he had been right.

Later theories of aromaticity, developed with the help of quantum mechanics, explained why rings of six carbon atoms are special and rings of eight are not. Cyclooctatetraene is now known to adopt a non-planar tub shape, and it remains a classic example in teaching these ideas.

Why it matters

Cyclooctatetraene is still used in chemistry courses to explain aromaticity, a concept vital to designing drugs, dyes and plastics.

About Richard Willstätter

Nobel-winning chemist who unravelled chlorophyll and the colours of flowers, refused to convert, resigned his chair in protest at antisemitism and died a refugee in Switzerland.

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Richard Willstätter

1872–1942 · Organic chemistry

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