Question

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

Answer 1

Hint: 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
 $n={\displaystyle \frac{4}{4}}$
 $n=1$
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
 $n={\displaystyle \frac{8}{4}}$
 $n=2$
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
 $n={\displaystyle \frac{4}{4}}$
 $n=1$
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.