Antiaromatic Compounds and Antiaromaticity

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18 thoughts on “Antiaromatic Compounds and Antiaromaticity”

1. How do we that if a compund is flat or not?

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   1. It’s generally a good assumption that if a molecule is cyclic, has 6 pi electrons, and is conjugated all around the ring, that it will arrange itself to be flat.
      Some antiaromatic compounds with 4- and 5- membered rings can’t really distort themselves out of flatness (too much ring strain!) , but larger ones (like cyclooctatetraene) can, and for that reason, cyclooctatetraene is considered to be “non aromatic” instead of antiaromatic.

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2. sorry, of course the azirinium cation!

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   1. That would be an interesting species to try to make. 6-electron nitrogen species are extremely unstable, but that one might have a shot. Do you know if anyone has tried making it?

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3. Interestingly, the 2H-Azirine is actually an aromatic compound! (flat, 2 pi electrons, cyclic, conjugated)

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4. One of the rings is aromatic only, the one with the three double bonds, the other ring as it is not conjugated or flat is simply non aromatic. Even if only one ring in a poly cyclic compound is aromatic the entire thing is considered aromatic.

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5. In my aromatic compounds test , In a question numbers of aromatic compounds are asked . In that dialin was considered as aromatic compound (in jee adv 2017 ) . Why they considered it as aromatic if this has not all atoms conjugated and not obey huckels rule also ?

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6. thank you for all this info!! I have a question about cyclobutadiene though, can we say it’s anti-huckel? and the formula 4n, does n represent the number of double bonds? or the number of pi bonds only? just a little confused on that part

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7. Hey, where’d you come up with the value of 18 kcal/mol for the energy difference between planar and tub shaped COT? If I remember right, the bond-alternated structure of planar COT is more like 12 kcal/mol above the tub. The octagonal structure is about 2 kcal/mol higher than that.

   These values can be deduced based on the old Paquette kinetics measurements for inversion and bond-shifting, or more directly from the photoelectron spectroscopy.

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8. On the cyclopentadiene, what does the + charge indicate? How does that compare to a cyclopentadiene with a negative charge?

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   1. The + charge indicates that one carbon is a carbocation. That is, a carbon having only 3 bonds, 6 valence electrons, and an empty lone pair.

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9. Hi! How would be know that cyclooctatetrene is flexible and therefore nonplanar? Are there other molecules that are non aromatic because of their flexibility?

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   1. Well, one way would be to make a model. Models are fairly reliable for determining such things. If you make a model of cyclooctatetraene you will see that it’s pretty floppy and can form a tub shape quite easily.
      As for other examples… well, here’s one: “The Great Escape” From Antiaromaticity… https://pubs.acs.org/doi/abs/10.1021/ja0291991

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10. There’s a typo: “Cyclobutadiene reacts with itself at -35 K to form a “dimer”, in an example of a Diels-Alder reaction.” Maybe -35 degrees C, but not negative Kelvin.

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    1. Ouch. Thanks for correcting that!

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