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 = \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.