I was just doing a simple question and for no reason, I drew the resonating structures of the compound as shown. I can see that some of the resonating structures seem to be aromatic while others seem to be non-aromatic And now I wonder..

  • Is aromaticity a property of a coumpound or any of its resonating structure? This compound has some Aromatic and one Anti-aromatic structure. Shall we call this compound aromatic or Anti-aromatic?

  • To compare stabilities of resonating structures, how will we use aromaticity and what will the stability order? enter image description here

  • 1
    $\begingroup$ There's nothing antiaromatic in there. Resonance not "resonating" structures aren't more or less stable, but are minor of major contributors to resonance hybrid, more or less relevant to depiction of the molecule. $\endgroup$ – Mithoron Mar 27 '19 at 23:31
  • $\begingroup$ So, we shall say that the compound is aromatic as per Aromaticity rules and structure b contributes the least to it as it promotes Anti-aromaticity? $\endgroup$ – Rajat Sansaniwal Mar 27 '19 at 23:39
  • 1
    $\begingroup$ No. As Mithoron pointed out, structure (b) is not anti-aromatic. $\endgroup$ – Zhe Mar 28 '19 at 2:22
  • $\begingroup$ Then, how will we compare the contributions to the hybrid? $\endgroup$ – Rajat Sansaniwal Mar 28 '19 at 4:40
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    $\begingroup$ The point that confuses you is to identify some of the mesomers as aromatic (though is an almost automatic way of thinking and is understandable, even useful). But none of the mesomers you draw is per se aromatic. It is just that a) and iv) describes aromaticity, they are among the most important. Ergo the hybrid will be more or less aromatic. $\endgroup$ – Alchimista Mar 28 '19 at 9:36

The reason for the question is that OP implicitly goes a step too further while identifying "aromatic" limiting forms.

This is of course quite natural, as we treat a limiting structure that is compatible with Huckel aromaticity as aromatic, ie we all more or less consciously or not replace three double bonds with a delocalized ring of electrons.

However not one individual limiting form is aromatic, each can be at most an aromaticity descriptor. In the example given, saying that a) and iv) are aromatic, although sensible, is logically incorrect, or at least involves the reasoning I have described above.

They are certainly important limiting forms as their linear combination stabilise the molecule which will therefore be more or less aromatic. But a mesomer cannot be aromatic by definition.*

I do not enter into the correctness of the antiaromatic part of the question, see comments above pointing to the fact that there is none.

Antiaromaticity is indeed confusing as for it should be taken as a rule for what

it does not happen.

However, this means that limiting forms leading to an antiaromatic flat system will have no weight or contribute marginally.

The concomitant occurrence of sensible limiting forms leading to aromaticity or formal antiaromaticity is not possible within the same flat cyclic system, as it would require a different number of electrons, so at least this point does not need to be discussed :)

*of course in a big molecule we can treat a far away, not resonating group as aromatic, but by doing this we implicitly account for all mesomers leading to its aromaticity.

  • $\begingroup$ I have understood till now that aromaticity has nothing to do with mesomers. It is the extra stabilisation of a compound, not its mesomers. It is related with a resonance hybrid(the reality) against the resonance structures. Am I correct? The doubt now is, if I am correct, how will we compare the contributions of each resonance structure. How will we define which structure contributes the most, and which the least and the reason therefore? $\endgroup$ – Rajat Sansaniwal Mar 28 '19 at 14:08
  • $\begingroup$ Does 'limiting form' stand for resonance structures? $\endgroup$ – Rajat Sansaniwal Mar 28 '19 at 14:09
  • $\begingroup$ As a, i,ii,iii and iv follow huckel's rule, and they are aromatic 'descriptors', can we say that the left out b contributes least to the resonance hybrid? $\endgroup$ – Rajat Sansaniwal Mar 28 '19 at 14:12
  • $\begingroup$ As usual! Yes assigning a minor weight to b) is anyway done quickly, as for Cl carries a + charge. Basically a) & iv) are the major contributors even without considering aromaticity. The fact that they are degenerate and lead to aromaticity is even a plus. Refer to the common criteria for qualitative ordering of mesimers. Yes, lim. forms stands for structure in resonance leading to the actual, real hybrid of resonance (which, after such a long discussion, will be much similar to a) resonating with iv)). $\endgroup$ – Alchimista Mar 28 '19 at 16:40
  • $\begingroup$ In b) there is a -ve charge on Cl(not +ve as u said) and there is one more covalent bond which increase its contribution in hybrid. This is leads to confusion to me:-). Clarify again please. $\endgroup$ – Rajat Sansaniwal Mar 28 '19 at 17:25

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