Aromaticity of conjugated cyclic ketones [closed]








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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?




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](https://i.stack.imgur.com/fMISI.jpg)](https://i.stack.imgur.com/fMISI.jpg)


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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