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If youre inclined to resist the popular assertion that chemistry is just applied quantum physics, you need do no more than invoke the notion of aromaticity. Of all chemistrys messy, ill-defined concepts, none is more so than this. It shows that chemistry is in some ways closer to sociology or zoology: populated by individuals molecules that convenience impels us to classify and group without being quite sure if our categories are sound.

The very word betrays its shaky status, being one of the more obvious misnomers of the field. It stems, of course, from the central role of the benzene ring, the hexagonal core of a slew of organic compounds notable for their pungency by the time August Kekul proposed the cyclic structure in 1865. Michael Faraday immediately noted the almond scent of benzene when he first isolated it in 1825. (He called it bicarburet of hydrogen, believing the ratio of carbon to hydrogen to be 2:1 because of the erroneous atomic weight then assigned to hydrogen.) Kekul presented the structure in an odd sausage format in his 1865 paper; only in 1872 did he show the familiar rings, noting that there were two equivalent configurations of the alternating single and double carboncarbon bonds.

The very word betrays its shaky status, being one of the more obvious misnomers of the field. It stems, of course, from the central role of the benzene ring, the hexagonal core of a slew of organic compounds notable for their pungency by the time August Kekul proposed the cyclic structure in 1865. Kekul presented this structure in an odd sausage format in his 1865 paper; only in 1872 did he show the familiar rings, noting that there were two equivalent configurations of the alternating single and double carboncarbon bonds.

It wasnt until the quantum theory of chemical bonding in the 1930s that benzenes electronic structure was clarified. Linus Pauling proposed that the two alternative Kekul structures oscillate rapidly back and forth in a resonance that accounts for the unusual stability of benzene: it is more stable than would be expected for any one of the Kekul structures. Erich Hckel, meanwhile, offered another description based on his notion of molecular orbitals, in which the hexagon of bonds between carbons is supplemented by the continuous rings of electrons from 2p orbitals overlapping above and below the plane of the atomic nuclei. These orbitals may sustain circulating electrical ring currents in an applied magnetic field, the effects of which account for the distinctive chemical shift of hydrogens attached to aromatic groups in NMR spectroscopy. That shift offers one way of assessing aromaticity.

The trouble is, its not unique in doing so. In fact, there are many such measures based, for example, on bond lengths, electronic structure, energetics and chemical reactivity. And they arent all consistent, so the extent to which a molecular fragment is aromatic depends on how you define it. Ostensibly tied to the quantum description of bonding, this putative feature of molecules is actually far more diffuse.

Arguments based on the symmetry properties of the electronic wavefunction that are needed to create a closed circuit of mobile electrons led to the initial view that aromaticity was confined to planar ring systems with 4n+2 electrons, but that limitation has long since been abandoned. For example, ring-like molecules with conjugated bonds that have the single twist of a Mbius strip attain aromatic-like stability if they have 4n electrons. Its now generally agreed that as well as bonds can partake in aromaticity. And it can appear in three-dimensional systems too, especially involving main-group elements such as clusters of boron atoms but also including the cage-like fullerenes. The latter illustrate the complications of an aromaticity criterion based on energetic stabilisation for how does one disentangle that from the strain energy of the curved framework?

With so little agreement about what is and is not aromatic, the concept is open to extensions and modifications and, some would say, abuses. Thus we see the appearance of doubly aromatic, hyperaromatic and superaromatic molecules. The latter, for example, was claimed for the macrocycle kekulene, made from 12 fused benzene rings in a hexagonal formation, until a recent single-molecule structural study using atomic force microscopy showed that the molecule does not after all contain two concentric, fully delocalised rings but is more like six independently aromatic units welded together.1

Still the putative variety of types of aromaticity proliferates: spherical and cubic aromaticity, transition-state aromaticity, homoaromaticity, and more. It has got to the point where some chemists disdain the whole concept as irredeemably vague and sloppy. Others instead cry Enough already! We should stop multiplying subclasses of aromaticity, says Miquel Sol of the University of Girona, and apply more self-discipline over the criteria used to identify it.2

Roald Hoffmann of Cornell University attributes aromaticity inflation to the natural human tendency to want our molecular children to be exceptional.3 Suppose we posit a molecule in which the occupied orbitals constitute a closed group then lets call it -aromatic! And yet, Hoffmann says, for some of these hypothetical molecules the supposed stabilisation applies to a structure that wouldnt survive an instant in air at room temperature.

Its not so much the hype that bothers him, but the damage done to this beautiful, eminently chemical idea that I can trace back a century and a half. Messy or not, aromaticity surely stands for something so we should be wary of debasing its currency.

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What is aromaticity? | Opinion - Chemistry World