August Kekulé and the Carbon Ring Structure

August Kekulé

On July 13, 1896, German organic chemist Friedrich August Kekulé passed away. Being one of the world’s leading chemists of his time, he is best known for devising the ring structure of carbon atoms in organic molecules and became the principal founder of the theory of chemical structure.

August Kekulé was born on September 7, 1829 in Darmstadt as son of a civil servant. After graduating from secondary school, in 1847 he attended the University of Giessen. At first, he wanted to study architecture but was convinced to continue with chemistry by the lectures of Justus von Liebig. After graduating, he took postdoctoral fellowships in Paris, Chur, Switzerland, and London. Shortly after, Kekulé was appointed full professor at the University of Ghent and later on at Bonn, where he remained for the rest of his career. Kekulé was the principal formulator of the theory of chemical structure and in his work, he was highly influenced by the work of Williamson, Edward Frankland, William Odling, Auguste Laurent, and Charles Adolphe Wurtz. The famous theory covers three main parts, the idea of atomic valence, the ability of carbon atoms to link to each other and the determination of the bonding order of all of the atoms in a molecule. The theory explained by Kekulé, provided a whole new perspective and clarity on analytic and especially synthetic work. Also, this contribution be Kekulé paved the way for organic chemistry.

Kekulé’s idea of assigning certain atoms to certain positions within the molecule, and schematically connecting them was mainly based on chemical reactions he observed. One of the main ideas about his structural chemistry was that the number of valences of a given element was invariant. However, numerous exceptions followed and it was then found out that valences were fixed at certain oxidation states. Periodic acid for example could, according to Kekulé’s structure theory, be represented by the chain structure I-O-O-O-O-H. The modern structure of periodic acid has all four oxygen atoms surrounding the iodine in a tetrahedral geometry.

To one of Kekulé’s biggest contributions to chemistry belongs his work on the structure of benzene. His first paper on the topic was published in 1865 and in it, he suggested that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds. After this work by Kekulé, a new understanding of benzene, and hence of all aromatic compounds, proved to be so important for both pure and applied chemistry after 1865 that in 1890 the German Chemical Society organized an appreciation in Kekulé’s honor, celebrating the twenty-fifth anniversary of his first benzene paper. There, Kekulé spoke of the creation of the theory and he said that he had discovered the ring shape of the benzene molecule after having a reverie or day-dream of a snake seizing its own tail. This vision, he said, came to him after years of studying the nature of carbon-carbon bonds.

At yovisto, you may be interested in a video lecture on Benzene and Aromaticity at the University of Berkeley by Professor Vollhardt.

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