Question

Which of the following is not aromatic?
(A) Benzene
(B) Naphthalene
(C) Pyridine
(D) $1,3,5 -$ heptatriene

Answer 1

Aromaticity of any compound is determined by applying Huckle’s rule which states that if any compound possesses $\left( {4n + 2} \right)\pi$ electrons which have capability to delocalize are known as aromatic compounds.

Complete Step By Step Answer:
In order to calculate the aromaticity of a compound we begin with determining the number of electrons present in it. After this, put the value of $\left( \pi \right)$ electrons in Huckle’s formula. If the final answer is a whole number then the compound must be aromatic.
Structure of benzene is:

From the above structure we find that there are a total $6\pi$ electrons present in benzene. On applying Huckle’s rule we get,
$\left( {4n + 2} \right) = 6$
$4n = 6 - 2$
After solving this we get
$n = \dfrac{4}{4}$
$n = 1$
Since, the value of $\left( n \right)$ is a whole number, benzene is an aromatic compound.

Structure of Naphthalene is:

From the above structure we find that there are a total $10\pi$ electrons present in naphthalene. On applying Huckle’s rule we get,
$\left( {4n + 2} \right) = 10$
$4n = 10 - 2$
After solving this we get
$n = \dfrac{8}{4}$
$n = 2$
Since, the value of $\left( n \right)$ is a whole number, naphthalene is an aromatic compound.

Structure of pyridine is:

From the above structure we find that there are a total $6\pi$ electrons present in pyridine. On applying Huckle’s rule we get,
$\left( {4n + 2} \right) = 6$
$4n = 6 - 2$
After solving this we get
$n = \dfrac{4}{4}$
$n = 1$
Since, the value of $\left( n \right)$ is a whole number, pyridine is an aromatic compound.

Structure of $1,3,5 -$ heptatriene is:

From the above structure we see that it is not a cyclic structure. therefore $1,3,5 -$ heptatriene fails to fulfill the basic requirement of aromaticity. Hence, $1,3,5 -$ heptatriene is not aromatic.
$\Rightarrow$ $1,3,5 -$ heptatriene is not an aromatic compound. Therefore option $\left( {iv} \right)$ is the correct option.

Note:
Due to the presence of aroma or pleasant smell from the compound they are known as aromatic. According to Huckle’s rule a compound is said to be aromatic if it satisfies the basic requirement like compound must be planar, cyclic, and have $\left( {4n + 2} \right)\pi$ delocalized electrons.