Which one of the following is not aromatic?

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(C) compound is aromatic if it follows $H$ückel's rule: it must be planar,cyclic,fully conjugated,and contain $(4n + 2) \pi$ electrons.$A$. Cyclopenta

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Cyclopentadienyl cation

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(C) compound is aromatic if it follows $H$ückel's rule: it must be planar,cyclic,fully conjugated,and contain $(4n + 2) \pi$ electrons.$A$. Cyclopentadienyl anion has $6 \pi$ electrons $(n=1)$,so it is aromatic.$B$. Benzene has $6 \pi$ electrons $(n=1)$,so it is aromatic.$C$. Cyclopentadienyl cation has $4 \pi$ electrons $(n=1)$,which follows the $4n$ rule,making it anti-aromatic.$D$. Cycloheptatrienyl cation has $6 \pi$ electrons $(n=1)$,so it is aromatic.Therefore,the cyclopentadienyl cation is not aromatic.

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