150^th Anniversary of the Kekulé Benzene Structure

There are more than 200 possible isomers for a molecule with the empirical formula C₆H₆. 150 years ago, the structure of benzene was a matter of some debate, and finding a model that could be justified with the available physical and chemical data was difficult, not least as some of the data (e.g., that benzene and toluene both had two isomers) was found to be false. The postulated structures included straight-chain diallenes or fused cyclopropane rings.

Alan Rocke, Case Western Reserve University, Cleveland, OH, USA, tells the story of August Kekulé (1829–1896), who was one of the early pioneers of structural organic chemistry. The cyclohexatriene structure of benzene was said to have come to Kekulé as he dozed in front of the fire and dreamed of a snake biting its own tail. He originally reported the structure in 1885, in a rather hesitant paper in which he did not explicitly state that benzene was a symmetrical molecule with a hexagonal form, but rather simply a closed chain. In 1866, Kekulé published another paper that firmly stated the structure of benzene, and this was a turning point in the development of multiple areas of chemistry, from aromatic chemistry (and the drug and dye industries) to quantum chemistry