Explain why the following systems are not aromatic?

Answer

Lack of delocalisation of `(4n+2)pi` electrons in the cyclic system.

Explanation

For the given compound, the number of `(pi)`-electrons is 6. By Huckel’s rule,
4n + 2 = 6
4n = 4
n = 1
For a compound to be aromatic, the value of n must be an integer (n = 0, 1, 2…). Since the value of n is an integer, the given compound is aromatic in nature.

(ii) For the given compound, the number of π-electrons is 4. By Huckel’s rule,
4n + 2 = 4
4n=2
`n=(1)/(2)`
For a compound to be aromatic, the value of n must be an integer (n = 0, 1, 2…), which is not true for the given compound. Hence, it is not aromatic in nature.
For the given compound, the number of π-electrons is 8.
By Huckel’s rule, 4n+2=8
4n=6
`n=(3)/(2)`
For a compound to be aromatic, the value of n must be an integer (n = 0, 1, 2…). Since the value of n is not an integer, the given compound is not aromatic in nature.