I was going through this site looking for some answer on aromatic character, definition and other ways of judging an aromatic compound, when is tumbled upon this question:

Are p‐xylylene and fulvene aromatic?

There, an answer stated that because there is cross conjugation in the structure due to which it is non aromatic. I doubt if that is true (may even be the reason for the 2 downvotes) since we have squaric acid:

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which is cross conjugated and aromatic. But I am never sure about organic chemistry, because I don't actually guarantee to know all rules for aromaticity, so i would just like to confirm. Does cross conjugation lead to non-aromaticity?


1 Answer 1


Aromaticity, and its antonym too, are the properties of simple cyclic systems of alternating single and double bonds. The use of the term elsewhere is an abuse of language.

That said, in practice people tend to apply the word "aromatic" to all kinds of systems. One just has to remember, though, that it is not a clearcut black-or-white characteristic and not a precise measure of anything. King Richard I of England was said to have the heart of a lion. That's a metaphor. When someone calls, say, naphthalene aromatic, it is the same thing.

Now to the point: is that non-rigorous use of "aromatic" justified when applied to cycles with side chains? The answer is: yes or no, depending on the particular system. You may look up the bond length, or compare the stability via enthalpies, or (if neither of these is available) calculate the orbital energies and see how it all turns out. Indeed, squaric acid is not really aromatic in that vague sense, but its dianion definitely is. So are the dianions of its triangular, pentagonal, and hexagonal siblings, known as deltic, croconic, and rhodizonic acids. On the other hand, 1,4-benzoquinone does not look aromatic at all.

Nature is under no obligation to give us a rule of thumb for deciding aromaticity just by looking at the formula. That such rule exists for simple cycles is a rare and lucky coincidence.

So it goes.

  • $\begingroup$ The aromatic character of the carbonyl ring anions is one factor that makes the neutral compounds relatively strong acids. Croconic acid, in particular, has a second deprotonation constant approaching that of sulfuric acid, without the use of halogen or cyano functions. $\endgroup$ Commented Apr 4, 2023 at 10:55
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    $\begingroup$ Yes, that's pretty impressive for an organic acid that does not even have a -COOH group. $\endgroup$ Commented Apr 4, 2023 at 12:27
  • $\begingroup$ Ok, so i get what you are trying to say, but just another thing to be sure of, is it possible for a monocyclic planar molecule with 4n+2 π electrons to be non aromatic just because it does not have continuous conjugation (and rather has cross conjugation)? $\endgroup$
    – Ham Lemon
    Commented Apr 4, 2023 at 13:05
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    $\begingroup$ Why, sure, the example from your very question will do: fulvene. $\endgroup$ Commented Apr 4, 2023 at 16:35

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