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Are the following structures aromatic?

p‐xylylene and fulvene

I have not seen this type of question anywhere so I do not know the answer. This is just a problem that has been roaming around in my head that I may have seen somewhere.

My approach: since aromaticity leads to stability the carbon-carbon bond in both structure should be polar leading to aromaticity like this:

p‐xylylene and fulvene — ionic form

But this is just a thought and I do not know if it is right. Is any of the two structures aromatic?

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To decide if a molecule is aromatic or not you must know some rules-

  1. The Molecule Must Be Cyclic and the system should be conjugated.

  2. The molecule must follow Huckle's rule i.e. it should have (4n+2) Pi Electrons participating in resonance.

    3.The molecule must be planar.

For details see-https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry/Map%3A_Organic_Chemistry_(McMurry)/Chapter_15%3A_Benzene_and_Aromaticity/15.03_Aromaticity_and_the_Huckel_4n___2_Rule

Now if you try to draw the resonance structure for your 1st molecule then you will see that 4 electrons are participating in resonance at a time. So the system is not fully conjugated. It will be a cross conjugated system.So both the structures you have mentioned are non aromatic.

Stability is a result of aromaticity. You can not comment that if a compound is stable then it is due to aromaticity.

Here is an image showing resonance in para xylylene.

Resonance in para xylylene

It can be seen from the resonating structures that some of the resonance structures are not aromatic making the molecule non-aromatic in total.

Any recommendations on making the image better are welcomed.

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  • $\begingroup$ I still do not understand why the overall molecule in 1st problem is not conjugated. Can you please elaborate on that. $\endgroup$
    – Groverkss
    Commented Aug 26, 2018 at 17:17
  • $\begingroup$ I have edited the answer if its still unclear do let me know. $\endgroup$
    – Pan
    Commented Aug 26, 2018 at 19:01
  • $\begingroup$ It is clear now but i think the 3rd structure should be the most contributing structure. Since that structure is highly contributing, shouldn't the molecule exhibit its properties? $\endgroup$
    – Groverkss
    Commented Aug 26, 2018 at 20:45
  • $\begingroup$ Molecule will exhibit the properties of the resonance hybrid and not of any one of its contributing structures. $\endgroup$
    – Pan
    Commented Aug 27, 2018 at 3:34
  • $\begingroup$ @Groverkss This will give you the most appropriate idea on it chemistry.stackexchange.com/questions/22100/… $\endgroup$
    – Pan
    Commented Aug 27, 2018 at 3:35

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