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I'm curious if there are any aromatic systems that don't contain any rings. I just wonder and want to know if there is any exception to the aromatic system.

I know the definition of aromaticity is that it must have a ring but my friend told me there are some aromatic molecules that don't have a ring.

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    $\begingroup$ The definition of an aromatic system is that it has one or more rings $\endgroup$
    – Waylander
    Commented Sep 26, 2022 at 13:28
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    $\begingroup$ That depends on what kind of aromaticity you'd talk about. What is commonly know as aromaticity, precludes something like that. That are some extensions of the concept on other systems, though. $\endgroup$
    – Mithoron
    Commented Sep 26, 2022 at 13:30
  • $\begingroup$ chemistry.stackexchange.com/questions/32354/… $\endgroup$
    – Mithoron
    Commented Sep 26, 2022 at 13:54
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    $\begingroup$ The guanidinium cation wants to be aromatic, definitions be darned! $\endgroup$ Commented Dec 12, 2023 at 19:59

1 Answer 1

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The word "aromatic" is most commonly used for systems that do have rings, but an enhanced resonance stabilization with delocalized $\pi$ bonding can occur without rings whether we apply the label "aromatic" or not.

Carbon dioxide is actually the first of a series of linear molecules having the formula $\ce{C_nO2}$ or $\ce{O=C=C=...=C=O}$, in which all the pairs of neighboring atoms are joined by double bonds and thus we have a linear molecule.

If we draw out the molecular orbitals of this structure we discover that the molecule will have a closed-shell structure when there are $4n$ $\pi$ electrons (not $4n+2$), and carbon dioxide with eight ($n=2$) has all the hallmarks of a system with extra $\pi$ electron stabilization and delocalization (see here for a discussion). Similarly, the $12\pi$ member carbon suboxide, $\ce{C3O2}$, formally has cumulated double bonds: but its carbon-carbon and carbon-oxygen bond lengths are shorter than those in ethylene and formaldehyde respectively. Carbon suboxide is prone to polymerization but can be kept as molecular $\ce{C3O2}$ under carefully controlled, dark conditions. By contrast, ethylene dione, $\ce{C2O2}$, with an unfavorable electron count for forming a closed shell, decomposes on a nanosecond time scale.

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    $\begingroup$ Resonance isn't, in general, enough for aromaticity. Did someone invent something like Y-aromaticity, but for cumulenes? $\endgroup$
    – Mithoron
    Commented Sep 26, 2022 at 21:37
  • $\begingroup$ @mithoron I haven't quite figured out the difference between enhancement of resonance stabilization with an optimal number of electrons but reduced stabilization with a wrong electron count, and aromaticity. Do tell. $\endgroup$ Commented Sep 26, 2022 at 21:45
  • $\begingroup$ @mithoron how about this: a chain of atoms has linear aromaticity if it (1) is cumulatively pi-bonded, (2) is linear or nearly so, (3) has all atoms in the chain sp hybridized, (4) contains 4n cumulated π electrons. The most stable linear carbon dioxides, plus some other elatively stable cumulated species such as nitrous oxide or allenide ion, all follow these rules. Makes as much sense as Y aromaticity to me. $\endgroup$ Commented Sep 26, 2022 at 22:56
  • $\begingroup$ Oh, well, you didn't really call something like that aromatic in the answer, so it's just that it is kinda a propos post, I guess. And yes, some Y-aromaticity makes about as much sense. You could expand the answer though. $\endgroup$
    – Mithoron
    Commented Sep 28, 2022 at 14:50

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