When we define the aromaticity of a compound, is it meant to describe the resonance hybrid of the compound, or for individual resonance structures of the compound?

I think the latter is wrong because in resonance structures all bonds are localized so there is no delocalisation of electrons, hence we cannot call it aromatic or non-aromatic. Also, resonance structures are hypothetical; they don't exist, so it doesn't seems reasonable to classify them as aromatic, non-aromatic, or anti-aromatic.

Yet, it's common to see some facts rationalised along the lines of "this resonance structure is aromatic and hence has a greater contribution in the hybrid". (For example, Is the aromaticity broken in some resonance structures of para-nitro-aminobenzene? where @ron begins his answer with the line some of the resonance structures in your figure are not aromatic. ) Is this usage of the term 'aromatic' correct / well-defined?

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    $\begingroup$ Aromaticity is nothing which is defined based on structure alone, especially not from diagrammatic structures. This includes, but is not limited to, resonance hybrid representations. As aromaticity is in itself a special form of resonance, and resonance structures (always more than one) are only (very, very crude) approximations to delocalisation, a single structure can never be called aromatic. $\endgroup$ Feb 5, 2022 at 0:17
  • $\begingroup$ @Martin-マーチン Agreed, but (in practice) there is a general trend of calling certain resonance structures aromatic to rationalise structure or other properties. For example, bond lengths in naphthalene / anthracene. I'm pretty sure this is a cheap & technically incorrect way of saying something else; but I can't quite put my finger on what that something else is. $\endgroup$ Feb 5, 2022 at 0:29
  • $\begingroup$ For example, I'm just not sure whether it's a cheap way of saying "this resonance structure must contribute more in order to reproduce the correct electronic structure of the compound, which in turn arises because it maximises stabilisation from aromaticity". My current thinking is something along those lines, but I'm not sure. [FWIW, I think this is actually a very good question.] $\endgroup$ Feb 5, 2022 at 0:33
  • $\begingroup$ There is some argument to be made that near equal bond lengths indicate near equal bonding situation, which in itself does imply a higher degree of delocalisation, which could be seen as more stabilisation, which could be inferred to as 'more aromaticity'. That is a structural argument which does bring molecular structure close to electronic structure. As far as approximations go, not a bad one. Anyway, for aromaticity (in resonance terms) you need at least two structures of (almost) equal weight, or one predominant structure with more structures of significant weight. $\endgroup$ Feb 5, 2022 at 0:44
  • $\begingroup$ chemistry.stackexchange.com/a/154063/9961 $\endgroup$
    – Mithoron
    Feb 9, 2022 at 18:55


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