April 2019 Limerick Project

Kekulé and Benzene

“The structure of resonant benzene
Found inception in Kekule’s daydream
As a snake seized its tail:
Vivid image availed
Him an insight once shrouded in smokescreen.”

The 14 April 2019 limerick retells a famous legend from chemical history: German organic chemist August Kekulé’s 1865 inspiration regarding the shape of the molecule benzene.

A major theme of chemistry is that the shapes (the structures) of molecules impact their behaviors (their functions); analyses of these structure-function relationships are part of many fields of chemistry research. With many compounds, their behaviors were observed in the laboratory before their chemical structures were known, and the paths to understand those structures include many interesting stories.

This story also provides a convenient overview of three types of chemical representations: empirical, molecular, and structural formulas.

“The structure of resonant benzene/
Found inception in Kekule’s daydream/
As a snake seized its tail…”
The molecule benzene contains six carbon atoms and six hydrogen atoms, as described by its molecular formula: C6H6. Before scientists understood this, they knew benzene’s empirical formula, which represents the lowest possible ratio of elements: here, CH. Since all that was known was that the molecule contained one carbon atom for every hydrogen atom, many possibilities were imagined for its shape.

According to legend, Kekulé had been pondering this question, when he had a daydream about a snake biting its tail. This inspired his idea of a cyclic compound, one in which carbon atoms formed a ring, instead of connecting to one another in a linear chain.

We now represent benzene as existing in a hexagonal shape, as succinctly shown via its structural formula. After Kekulé’s revelation, further study of benzene revealed an interesting bonding pattern called resonance, which accounts for benzene’s unusual stability.

“Vivid image availed/
Him an insight once shrouded in smokescreen.”
Kekulé later popularized the dramatic nature of his insight, writing: “One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke… I spent the rest of the night in working out the consequences of the hypothesis.”

As with many of the stories behind scientific discoveries, debates have arisen as to the veracity of the details. That said, the last line of this limerick is a final allusion to the chemical legend, since it is generally recounted that Kekulé had his daydream in front of the fireplace.