Why Aromaticity Is a Suspicious Concept? Why?

Abstract

From time to time I have the opportunity to give lectures on topics related to aromaticity. Quite often in these occasions I get comments from the audience complaining about the fact that aromaticity is not a well-defined concept. My usual answer is that the most fruitful concepts in chemistry share the same lack of strict definition (Grunenberg, 2017). In one of these occasions, the comment was formulated by someone who give a talk the day before justifying all the results he/she obtained using the concept of hyperconjugation. His/her comment was a little bit irritating to me because, in a way, he/she was saying I am a serious scientist because I am working with rigorous concepts like hyperconjugation whereas you are a kind of pseudoscientist playing with floppy concepts like aromaticity. Was he/she right? I do not think so.

Conjugation involves interactions (electron delocalization) between π-orbitals, although its definition can also be extended to p-orbitals to cover lone pair interactions with the π-system. Hyperconjugation accounts for the interaction between two orbitals with π-symmetry where one or both of them come from a saturated moiety (Mulliken, 1939; Mulliken et al., 1941). It can also be defined as the interaction between the orbitals involved in a σ-bond (usually C–H or C–C) with those related with an adjacent π-bond (usually C=C) or another σ-bond. Aromaticity is conjugation (and in some cases hyperconjugation) that generates closed two- and three-dimensional electronic circuits. Conjugation, hyperconjugation, and aromaticity lead to stabilizing interactions that influence the geometry, electron density, dissociation energies or nuclear magnetic resonance properties among many other physicochemical observables. Despite their importance and widespread use, neither hyperconjugation nor aromaticity have a strict physical definition and, therefore, these properties cannot be experimentally directly measured. These two properties share the same origin that is stabilization due to electron delocalization. Indeed, differences between these two concepts are minor as compared to similarities. Thus, the claim that one property is more rigorous than the other is totally unfounded.

The above-mentioned anecdote together with the existence of a series of papers (Balaban, 1980; Lloyd, 1996; Hoffmann, 2015) discussing the concept of aromaticity point out that aromaticity for some chemists is a controversial concept, while parent concepts like conjugation or hyperconjugation are not. Why? In the next paragraphs, I pointed out possible explanations to this fact and I propose ways of action to improve the prestige of this concept