The Many Guises of Aromaticity
Is hype debasing a core chemical concept?
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. The reactions involved were relatively easy, the products often stable and useful. Aspirin, TNT, mescalin, vanillin, and serotonin all contain an aromatic, benzenoid core.
As chemistry evolved, it was natural to seek other gauges of stability and the capacity to change while retaining the C6 core. Clothed in various measures, wonderfully expanding in scope, mental catnip for my fellow theoreticians, aromaticity, the concept, flourished. I will sketch its flowering, from the seminal paper of German chemist August Kekulé that proposed the correct structure of benzene 150 years ago, to those new measures.
Today, an inflation of hype threatens this beautiful concept. Molecules constructed in silico are extolled as possessing surfeits of aromaticity—“doubly aromatic” is a favorite descriptor. Yet the molecules so dubbed have precious little chance of being made in bulk in the laboratory. One can smile at the hype, a gas of sorts, were it not for its volume. A century and a half after the remarkable suggestion of the cyclic structure of benzene, the conceptual value of aromaticity—so useful, so chemical—is in a way dissolving in that hype. Or so it seems to me.