August Kekulé

Friedrich August Kekulé, later ennobled as August Kekule von Stradonitz, was a German organic chemist who dominated European theoretical chemistry from the 1850s until his death in 1896. He is credited as the main architect of chemical structure theory and is most famous for proposing the ring structure of benzene.

He went by August Kekulé his entire life, never using his first given name. The French acute accent over the second "e" was added by his father during the Napoleonic occupation of Hesse, to ensure French speakers pronounced the third syllable. After being ennobled by the Kaiser in 1895, he dropped the accent and became August Kekule von Stradonitz.

Born in Darmstadt, the capital of the Grand Duchy of Hesse, to a civil servant, Kekulé entered the University of Giessen in the fall of 1847 intending to study architecture. After hearing Justus von Liebig’s lectures in his first semester, he switched to chemistry. While a student, he was called as a witness in the trial of a servant named Stauff, concerning the partially charred corpse of Countess Emile von Görlitz. Von Liebig had testified that the suspected spontaneous combustion from alcohol was chemically impossible. A key piece of evidence was a ring showing two snakes—one gold, one platinum—biting each other’s tails, a motif Kekulé would later dream about.

After four years at Giessen, brief military service, and temporary assistantships in Paris, Chur (Switzerland), and London—where Alexander Williamson strongly influenced him—he received his doctoral degree from Giessen in the summer of 1852.

In 1856, Kekulé became a Privatdozent at Heidelberg. He was hired as a full professor at the University of Ghent in 1858, then moved to the University of Bonn in 1867, where he stayed for the rest of his career. Building on the work of Williamson, Charles Gerhardt, Edward Frankland, William Odling, Auguste Laurent, Charles-Adolphe Wurtz, and others, Kekulé was the main formulator of chemical structure theory (1857–58). The theory relied on atomic valence—especially carbon’s tetravalence, which Kekulé announced in late 1857—and the ability of carbon atoms to link to each other, announced in a paper published in May 1858. Archibald Scott Couper independently arrived at the idea of carbon self-linking (his paper appeared in June 1858) and was the first to use lines to represent bonds between atoms. The theory gave organic chemists a new clarity, guiding both analysis and synthesis, and sparked explosive growth in the field. Early structural investigators included, besides Kekulé and Couper, Frankland, Wurtz, Alexander Crum Brown, Emil Erlenmeyer, and Alexander Butlerov.

Kekulé assigned atoms to specific positions within a molecule and connected them using what he called “affinity units” (now valences or bonds), based largely on chemical reactions rather than direct instrumental methods like X-ray crystallography, which did not yet exist. Some chemists, notably Hermann Kolbe, criticized these structural formulas as unproven. But most followed Kekulé’s lead in developing classical structure theory, which was later modified after the discovery of electrons in 1897 and the development of quantum mechanics in the 1920s.

A key part of Kekulé’s version was the idea that an element’s valence was invariant. This had many exceptions and was later replaced by the concept of fixed valences at certain oxidation states. For example, Kekuléan theory represented periodic acid as I-O-O-O-O-H, whereas the modern structure has four oxygen atoms around iodine in a tetrahedral geometry.

Kekulé’s most famous work was on benzene. In 1865, while still in Belgium, he published a paper in French suggesting a six-membered carbon ring with alternating single and double bonds. He published a longer German paper on the same subject the following year. The empirical formula for benzene had long been known, but its highly unsaturated structure was difficult to determine. Couper in 1858 and Joseph Loschmidt in 1861 had proposed structures with multiple double bonds or rings, but too little evidence existed to decide. By 1865, more evidence was available, especially regarding the relationships of aromatic isomers.