One of chemistry’s most crucial concepts is in crisis – can we fix it?

Molecular structures, computer artwork

In 1862, a chemist nodded off in front of a fire and began to dream. August Kekulé had been pondering the most pressing question in his field at the time: what was the chemical structure of a curious compound called benzene? As Kekulé slumbered, the atoms danced in his mind and organised themselves into a vision of a snake eating its own tail. That was it! The carbon atoms of benzene were joined together in a ring.

It may not have quite the drama of Archimedes running through the streets naked, but, in its own way, it was a eureka moment. Benzene turned out to be the archetype of a compound with a property called aromaticity, now recognised as among the most important ideas in chemistry. Instead of standard bonds, aromatic molecules contain dynamic, ring-shaped bonding that endows them with incredible stability and a raft of other handy properties. Two-thirds of known molecules are aromatic, including components of DNA and proteins, the building blocks of life.

Recently, though, the dream has turned into a nightmare. We have been discovering new kinds of aromaticity everywhere – to the point where there are dozens of competing definitions. It may be one of the most important concepts in chemistry, but no one can agree on what it means any more. It isn’t all bad news, though. The confusion has prompted chemists to think deeply about aromaticity and this has led to a raft of new ideas about how we can put it to use.

Chemistry is the science of atoms, bonds and molecules and the craft of cajoling those molecules to react with one …

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