Which of the following are not aromatic?

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(B) Key Idea: Conditions for a species to be aromatic are:$(I)$ The structure must follow $H$ückel's rule,i.e.,it must have $(4n+2) \pi$-electrons,whe

Vedclass · Accepted Answer

$B, E, F$

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(B) Key Idea: Conditions for a species to be aromatic are:$(I)$ The structure must follow $H$ückel's rule,i.e.,it must have $(4n+2) \pi$-electrons,where $n$ is an integer $(0, 1, 2, 3, \dots)$.$(II)$ The structure must be a planar cyclic ring.$(A)$ Cyclopentadienyl anion: It has $6 \pi$-electrons $(n=1)$ and is planar. Thus,it is aromatic.$(B)$ $1,2-$dihydronaphthalene: It is not fully conjugated and has $8 \pi$-electrons. Thus,it is not aromatic.$(C)$ Tropylium cation: It has $6 \pi$-electrons $(n=1)$ and is planar. Thus,it is aromatic.$(D)$ Phenanthrene: It has $14 \pi$-electrons $(n=3)$ and is planar. Thus,it is aromatic.$(E)$ Cyclopentadienyl cation: It has $4 \pi$-electrons ($n=1$ for $4n$ rule). Thus,it is anti-aromatic (not aromatic).$(F)$ Cyclopropenyl anion: It has $4 \pi$-electrons ($n=0$ for $4n$ rule). Thus,it is anti-aromatic (not aromatic).Therefore,species $(B)$,$(E)$,and $(F)$ are not aromatic. Hence,option $(b)$ is the correct answer.

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