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Consider the following compounds:

  1. Cyclopenta-2,4-dien-1-one
  2. Cyclopent-2-en-1-one

In both compounds, I can visualize a resonating structure in which the ring follows Huckel's rules by:

  1. The $\ce{C=O}$ $\pi$-electrons transfer to $\ce{C}$.
  2. The $\ce{C=O}$ $\pi$-electrons transfer to $\ce{O}$.

A test that I gave, gave 2 as aromatic but not 1, why?

Also can you recommend a lucid source with an explanation of aromaticity that has lots of examples?

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    $\begingroup$ You know Huckel's rules? There's no reason to think any of the two would be aromatic. $\endgroup$ – Mithoron May 22 at 19:49
  • $\begingroup$ Possibly the second compound is misprinted. Cycleprop-2-en-1-one would have an aromatic contribution. $\endgroup$ – Oscar Lanzi May 23 at 1:19
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Cyclopent-2,4-dien-1-one is not aromatic as you suggested by the resonance structure (see A on the following diagram). That resonance is forbidden and does not exist, ever (I marked it with a red cross). Also, Note that the conjugated $\mathrm{sp^2}$ carbons of the original structure are not cyclic. Nonetheless, its allowed resonance is given in B, and therefore, cyclopent-2,4-dien-1-one is considered to be antiaromatic (Ref.1) due to a significant polar resonance structure (the carbocation is formed at the carbonyl carbon; see B on the diagram):

Cyclopenta-2,4-dien-1-one

It is a very unstable compound and reacts with itself in a Diels-Alder reaction to give a more stable dimer, resemble cyclopentadiene. It is noteworthy that all attempts to synthesize cyclopenta-2,4-dien-1-one have been failed and, yielded only a dimerization product (see C on the diagram).

Cyclopent-2-en-1-one is, on the other hand, not aromatic or antiaromatic, but a $\alpha,\beta$-unsaturated cyclic ketone, which is also very stable.

Note: You may read https://nptel.ac.in/courses/104103071/module7/lec17/2.html, https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Arenes/Properties_of_Arenes/Aromaticity, and https://www.masterorganicchemistry.com/2017/02/23/rules-for-aromaticity/ to learn about aromaticity.

References:

  1. R. Pal, S. Mukherjee, S. Chandrasekhar, T. N. Guru Row, “Exploring Cyclopentadienone Antiaromaticity: Charge Density Studies of Various Tetracyclones,” J. Phys. Chem. A 2014, 118(19), 3479–3489 (https://doi.org/10.1021/jp5010924).
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