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The structure of the benzene molecule

By the mid-19th century, the two chemists Kekule and Ladenburg tried to determine the structure of a well-known molecule, benzene.

Benzene

Benzene is a colorless liquid with a characteristic odor, irritating to skin and eyes, toxic and carcinogenic. Its molecular formula is $C_6H_6$

An experimental fact

By substitution of an H atom of benzene with chlorine, we find only one single $C_6H_5Cl$. Kekule and Ladenburg have concluded that all $H$ atoms of benzene are "equivalent"

Ladenburg imagined benzene as a triangular prism where actually all $H$ atoms are equivalent:

The structure of Kekule

Kekule imagined (in a dream!) benzene as a hexagon with alternating single and double bonds where still all $H$ atoms are equivalent:

The confirmation of the structure of Kekule

Here is an actual reaction of benzene that the structure of Ladenburg does not explain: $C_6H_6$ $+$ $3H_2$ $\longrightarrow$ $C_6H_{12}$ Indeed, an alkane such as the single bonded "prismane" of Ladenburg does not add hydrogens, while the Kekule structure can explain the reaction:

Some disturbing facts

When Kekule was still alive, disturbing facts about its structure were presented that led half a century later to the notions of resonance or mesomerism .