# Aromaticity of conjugated cyclic ketones [closed]

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?

• You know Huckel's rules? There's no reason to think any of the two would be aromatic. – Mithoron May 22 '19 at 19:49
• Possibly the second compound is misprinted. Cycleprop-2-en-1-one would have an aromatic contribution. – Oscar Lanzi May 23 '19 at 1:19

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):

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.

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).