August Kekulé

August Kekulé was a highly influential German organic chemist.

He lived from September 7, 1829, to July 13, 1896. Kekulé became a central figure in European chemistry during the 1850s. He is best known for founding the theory of chemical structure. This theory explains how atoms are arranged within a molecule. His work provided a roadmap for scientists to understand and build complex substances. Without his theories, the field of organic chemistry would not have developed so rapidly.

Kekulé's theory of structure relies on the concept of atomic valence. Valence refers to the number of bonds an atom can form with others. A major part of his theory was the tetravalence of carbon. This means a single carbon atom can form four bonds. He also proposed that carbon atoms could link to one another to form chains. By understanding these connections, chemists could determine the specific bonding order of atoms. This allowed them to move from simple observation to purposeful chemical synthesis.

Kekulé did not have modern tools like X-ray crystallography to see molecules. Instead, he relied on "wet" chemistry, which involves observing liquid chemical reactions. He used evidence from how substances reacted to assign atoms to specific positions. He called these connection points "Verwandtschaftseinheiten," or affinity units. Today, we simply call these bonds. While some scientists, like Hermann Kolbe, criticized his lack of direct proof, most followed his lead. This approach became known as classical structure theory. It remained the standard until the discovery of electrons in 1897 and the rise of quantum mechanics in the 1920s.

Kekulé's career took him across many famous European universities. He was born in Darmstadt, the capital of the Grand Duchy of Hesse. He originally intended to study architecture at the University of Giessen. However, a lecture by Justus von Liebig convinced him to pursue chemistry instead. After earning his doctoral degree in 1852, he worked in Paris, Switzerland, and London. He served as a professor at the University of Ghent in 1858. In 1867, he joined the University of Bonn, where he worked until his death. In 1895, the Kaiser ennobled him, changing his name to August Kekule von Stradonitz.

His most significant breakthrough concerned the structure of benzene.

For years, the empirical formula for benzene was known, but its shape was a mystery. In 1865, Kekulé proposed that benzene was a six-membered ring of carbon atoms. He suggested these atoms were connected by alternating single and double bonds. He used the behavior of benzene derivatives to prove his idea. For example, he noted that adding one group to benzene produced only one isomer. Adding two groups produced three distinct isomers, which he called ortho, meta, and para. This specific pattern only made sense if the carbon atoms were arranged in a ring.

Kekulé famously attributed his discovery to a vivid dream. He claimed he saw a snake seizing its own tail, an ancient symbol called an ouroboros. This vision helped him realize the possibility of a ring-shaped molecule. He also recalled a vision of dancing atoms while riding an omnibus in London in 1855. While some historians wonder if these stories were true or creative retellings, they highlight his unique way of thinking. To solve a criticism regarding the symmetry of his ring, he suggested in 1872 that the molecule oscillates. This means the single and double bonds constantly switch positions, making all six bonds equivalent.

Kekulé's scientific legacy is visible in the success of his students. Three of his former students went on to win Nobel Prizes in Chemistry. These were van 't Hoff in 1901, Fischer in 1902, and Baeyer in 1905. His impact on the study of aromatic compounds remains fundamental to modern science. Today, a large monument of Kekulé stands in front of the former Chemical Institute at the University of Bonn. It remains a popular landmark for students in the city.