By the 1860s chemists knew benzene had the formula C6H6, and that formula was a puzzle. Six carbon atoms with only six hydrogens did not fit any chain-like arrangement then in use. The German chemist Friedrich August Kekulé (1829–1896) answered the puzzle in 1865 with a closed ring of six carbons.
Kekulé was born in Darmstadt on September 7, 1829, studied at the University of Giessen, and worked at Heidelberg, Ghent and Bonn. He died in Bonn on July 13, 1896.
Carbon makes four bonds and can form chains
Kekulé’s benzene work rested on two ideas from the late 1850s. The first was that carbon is tetravalent. Valence is an atom’s combining power, and carbon forms four bonds, as in methane (CH4) and carbon tetrachloride (CCl4). The second was that carbon atoms can bond to one another and form chains. Together these ideas let chemists draw a molecule as a connected skeleton of atoms. In 1859 he began using such diagrams to show carbon’s four bonds and its ability to form chains.
The Scottish chemist Archibald Couper reached similar conclusions independently, and he did so slightly earlier.
The benzene ring
Kekulé proposed that benzene is a hexagon of six carbon atoms, each carrying one hydrogen, with single and double bonds alternating around the ring. Each carbon then has four bonds: one to hydrogen, one to each neighbor, and one extra from the double bond. The structure also fit the evidence. Replacing any one hydrogen gives only one product, so all six carbons must be equivalent.
He later said the idea came to him in a dream of a snake seizing its own tail. It is a famous story, but it comes from his own recollection many years afterward.
A problem he had to fix
The alternating-bond picture predicted two different compounds when two neighboring carbons each carry a substituent. One would have the substituents on a double-bonded pair and the other on a single-bonded pair. Chemists found only one. In 1872 Kekulé suggested that the two arrangements swap places so fast that they cannot be separated.
Modern chemistry replaces that swapping with resonance. The six electrons in the double bonds are shared around the whole ring, so every carbon–carbon bond is the same length and in between a single and a double bond. This sharing explains why benzene is unusually stable and does not react like an ordinary alkene.
Why students still learn it
The hexagon with a circle or alternating lines is the standard way to draw aromatic compounds. Textbooks use Kekulé’s reasoning, which counts isomers to test a proposed structure, as a model of how structural arguments work. His work on tetravalent carbon is the basis of organic structural formulas. He received the Copley Medal in 1885.