Which of the following species is aromatic?

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

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Cyclopentadienyl anion

Vedclass · Answer

(A) compound is aromatic if it follows $H$ückel's rule: it must be cyclic, planar, fully conjugated, and contain $(4n + 2) \pi$ electrons, where $n$ is an integer $(n = 0, 1, 2, ...)$.$1$. Cyclopentadienyl anion: It has $2$ double bonds ($4 \pi$ electrons) and a lone pair on the carbon atom ($2 \pi$ electrons), totaling $6 \pi$ electrons. This follows the $(4n + 2)$ rule $(n=1)$, making it aromatic.$2$. Cycloheptatrienyl cation (tropylium ion): It has $3$ double bonds ($6 \pi$ electrons) and an empty $p$-orbital on the carbocation. This also follows the $(4n + 2)$ rule $(n=1)$, making it aromatic.$3$. Cyclopentadienyl cation: It has $4 \pi$ electrons, following the $4n$ rule $(n=1)$, making it anti-aromatic.$4$. Cycloheptatrienyl anion: It has $8 \pi$ electrons, following the $4n$ rule $(n=2)$, making it anti-aromatic.Note: Since both $(A)$ and $(B)$ are aromatic, in standard chemistry examinations, the Cyclopentadienyl anion is the most common example of an aromatic ion.

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