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(B) For a compound to be aromatic, it must satisfy $H$ückel's rule:$(I)$ The molecule must be cyclic.$(II)$ The molecule must be planar.$(III)$ The mo

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Cyclooctatetraene

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(B) For a compound to be aromatic, it must satisfy $H$ückel's rule:$(I)$ The molecule must be cyclic.$(II)$ The molecule must be planar.$(III)$ The molecule must be fully conjugated (all atoms in the ring must have an unhybridized $p$-orbital).$(IV)$ The molecule must have $(4n+2) \pi$ electrons, where $n = 0, 1, 2, ...$.Analysis of the given compounds:$(A)$ Cyclopropenyl cation: Cyclic, planar, conjugated, and has $2 \pi$ electrons $(n=0)$. It is aromatic.$(B)$ Cyclooctatetraene: It has $8 \pi$ electrons ($4n$ system, $n=2$). Furthermore, it adopts a non-planar 'tub' shape to avoid anti-aromaticity. Thus, it is non-aromatic.$(C)$ Cyclopentadienyl anion: Cyclic, planar, conjugated, and has $6 \pi$ electrons $(n=1)$. It is aromatic.$(D)$ $6-$(dimethylamino)fulvene: This is a substituted fulvene derivative that exhibits aromatic character due to the contribution of a dipolar resonance structure where the five-membered ring becomes a cyclopentadienyl anion ($6 \pi$ electrons).Therefore, cyclooctatetraene is not aromatic.

From the following compounds, choose the one which is not aromatic.

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