Perspective The Many Guises of Aromaticity

Aromaticity is one of the core concepts of organic chemistry. The idea began as a descriptor of the special stability of the ring of six carbons, benzene, C6H6. And importantly, of the ability of that ring to be transformed by chemically substituting the hydrogens attached to it. Aromaticity provides at least three motivations, first a search to better define the concept, hoping against hope that there is a unique measure of this elusive property. Second, one wants to explore all of its experimental manifestations. This has changed with time, as our tools have, we could not measure internuclear separations and chemical shifts in 1900. People have been inspired by aromaticity as a design principle to make ever more interesting molecules. The concept will survive because in its strong form, in the shape of benzene and other smaller ring systems with delocalized bonds, it singles out a group of molecules whose kinetic persistence and thermochemical stability go hand in hand. Another reason the concept will survive comes from its inherently chemical changeable nature.