Question

Which of the following compounds are aromatic.
A.
B.
C.
D.

Answer 1

Aromatic compounds are defined as characterized by the presence of one or more ring which are uniquely stable structures which means the $\pi$(pi) electrons of molecules have strong bonding arrangements. The compound must follow these four conditions which are
-The aromatic compound must be a cyclic compound.
-The aromatic compound has planarity i.e. $s{p^2}$ hybridization.
-Delocalization of $\pi$ electrons are also present in aromatic compounds.
-The aromatic compound followed the Huckel’s rule of aromaticity i.e. $\left( {4n + 2} \right)\pi$ electrons.

Complete answer:
So, here in option (A), It is a cyclic compound and also has $s{p^2}$ hybridization in which planarity is present. There is $5\pi$ bond that is present in which $5 \times 2\pi$electrons are involved in the ring.
According to Huckel’s rule of aromaticity i.e. $\left( {4\pi + 2} \right)$ $\pi$ electron …… (i)
Put the value of n in the equation (i)
In which, $n = 0 \to \left[ {4\left( 0 \right) + 2} \right] = 2\pi$electron pair
$n = 1 \to \left[ {4\left( 1 \right) + 2} \right] = 6\pi$ electron pair
$n = 2 \to \left[ {4\left( 2 \right) + 2} \right] = 10\pi$ electron pair
Here $10\pi$ electrons are matched to the value of $\pi$ electron of this compound. So, this compound is an aromatic compound.
In the option (B), the given compound is cyclic compound and have $s{p^2}$ hybridization, the delocalization of $\pi$ electrons are also present and $4\pi$ bond occur which means $4 \times 2\pi$electron $= 8\pi$ electrons. According to Huckel’s rule of aromaticity i.e. $\left( {4n + 2} \right)\pi$electrons are not applied in this compound due to the value of Huckel’s rule are not equal or match with the $8\pi$ electron. So, this is not an aromatic compound.
In the option (C), these are not cyclic compound and $s{p^2}$ hybridization is absent, delocalization of $\pi$ electron may occur in which $2\pi$ bond and these have $2 \times 2\pi$ electron which means $4\pi$ electron and their value is not equal to the Huckel’s rule of aromaticity. So, this compound does not follow the conditions of aromaticity.
In the option (D), these are cyclic compounds and satisfy all conditions of aromaticity except one of them i.e. these do not follow the Huckel’s rule of aromaticity. The second five membered cyclic ring has an anion charge not cation. So, it loses aromaticity due to $8\pi$ electrons, it is not an aromatic ring.
Therefore, (A) is the correct option because this compound is an aromatic compound which follows all the properties of aromaticity.

Note:
Huckel’s rule of aromaticity is the rule which determines that the ring is planar which has aromatic properties in the molecule. The cyclic molecules are considered aromatic if it has $4n + 2\pi$ electrons.