Question

Which of the following is not aromatic?
A.Benzene
B.cyclooctatetraenyl dianion
C.tropylium ion
D.cyclopentadienyl cation

Answer 1

For a molecule to be aromatic, it should be cyclic, planar, must fulfil Huckel’s rule and must be conjugated. His rule states that if a cyclic, planar molecule has 4n+2 π electrons, it is considered To be aromatic.

Complete step by step answer:
Non-aromatic does not follow Huckel’s rule. are highly unstable and highly reactive.
A.Benzene is cyclic, planar and has 6 $\pi$ electrons thus fulfilling Huckel’s rule. It is therefore aromatic.

6 $\pi$ electrons
B.Cyclooctatetraenyl Dianion has 10 $\pi$ electrons thus fulfilling Huckel’s rule. It is cyclic as well as planar. It is therefore aromatic.

10 $\pi$ electrons
C.Tropylium Ion has 6 $\pi$ electrons. Thus it also fulfils Huckel’s rule. It is cyclic as well as planar. Therefore, it is also aromatic.

6 $\pi$ electrons
D.In Cyclopentadienyl cation, if a hydride anion $\left( {H - } \right)$ is removed from the
$sp3$ hybridized ring carbon, the cyclopentadienyl cation is formed, and hybridization of $sp3$ ring carbon is altered to $sp2$ .

4 $\pi$ electrons
Thus, the cyclic cyclopentadienyl cation is planar and possesses a cyclic uninterrupted $\pi$ electron cloud. However, it does not follow Huckel’s rule as it has 4 $\pi$ electrons in a conjugated system. Thus, it is antiaromatic.

Hence option D is correct.

Note:
Molecules may change shape, becoming non-planar to avoid the instability of anti-aromaticity and therefore breaking some of the π interactions.