Question

Which of the following is not an aromatic compound?
A)

B)

C)

D)

Answer 1

Follow the Huckel rule of aromaticity. A conjugated, cyclic planar compound having $\left( {4n + 2} \right){\text{ }}\pi$ electrons is aromatic.

Complete answer:
If the number of pi electrons in the compound is not equal to $\left( {4n + 2} \right)$ then the compound is non aromatic. For $n = 0,1,2,3...$ the value of $\left( {4n + 2} \right)$ is $4\left( 0 \right) + 2 = 2,4\left( 1 \right) + 2 = 6,{\text{ }}4\left( 2 \right) + 2 = 10,{\text{ }}4\left( 3 \right) + 2 = 14...$ respectively.
Hence, if the number of pi electrons in a compound is not equal to 2,6,10, 14… then it is not an aromatic compound. In the option A ) cycloheptatrienyl anion has 8 pi electrons. Thus the number of pi electrons in cycloheptatrienyl anion is not equal to 2,6,10, 14…
Hence, cycloheptatrienyl anion is not an aromatic compound.

Hence, the correct option is option A ).

Additional Information: The compounds/ions in options B ) and D ) have 6 pi electrons. They are cyclic, planar and conjugated systems of pi electrons. Hence, compounds/ions of options B ) and D ) are aromatic.
The ion of option C ) has 2 pi electrons. It is conjugated, cyclic and planar. Hence, it is aromatic in nature.

Note: While counting the number of pi electrons, each double bond corresponds to a pair of electrons. Each lone pair of electrons corresponds to a pair of electrons. The orbital in which the lone pair of electrons is present, should be parallel to the p orbitals containing pi electrons. If a molecule/ion contains $\left( {4n + 2} \right){\text{ }}\pi$ pi electrons, but it is non cyclic, non planar or non conjugated, then it is not an aromatic compound.