Which of the following are not aromatic?

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(D) To determine aromaticity,we use $H$ückel's rule: a compound is aromatic if it is cyclic,planar,fully conjugated,and contains $(4n + 2) \pi$ electr

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Cyclopentadienyl cation

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(D) To determine aromaticity,we use $H$ückel's rule: a compound is aromatic if it is cyclic,planar,fully conjugated,and contains $(4n + 2) \pi$ electrons.$A$. Benzene $(C_6H_6)$ has $6 \pi$ electrons $(n=1)$,so it is aromatic.$B$. Cyclooctatetraenyl dianion $(C_8H_8^{2-})$ has $10 \pi$ electrons $(n=2)$,so it is aromatic.$C$. Tropylium cation $(C_7H_7^+)$ has $6 \pi$ electrons $(n=1)$,so it is aromatic.$D$. Cyclopentadienyl cation $(C_5H_5^+)$ has $4 \pi$ electrons ($n=1$ in $4n$ rule),making it anti-aromatic. Thus,it is not aromatic.

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