Which compound amongst the following is not an aromatic compound?

Concept:

Rule for aromaticity - Huckel gave the rule to determine whether a given compound is aromatic or not is called Huckel's (4n+2)π electron rule.

Huckel's (4n+2)π e^- rule -

• This is also called the rule of aromaticity as it states whether a compound is aromatic or not.
• Huckel gave a few conditions, if the given compound follows all the conditions then it is said to be aromatic otherwise not. The conditions are-
  • Given molecule must be cyclic.
  • The given molecule must be a planar ring or atoms of the molecule must lie in the same plane.
  • The molecule must have conjugated pi bonds in its structure or the molecule must be fully conjugated.
  • The given molecule must have 4n+2 π electrons, where, n is either zero or any positive integer.

How to calculate (4n+2)π  electrons -

1. Calculate the no. of pi electrons present in the molecule
2. Put them equal to (4n + 2).
3. Calculate the 'n' value here.
4. If n is 0 or a positive integer, then the molecule is aromatic, otherwise not.

Calculation:

1. It is having a ring in its structure. So, it follows condition no. 1 of aromaticity.
2. It is having a planar ring, all pi bonds lie in the same plane. Hence, follow the second condition.
3. In its structure total π electrons are 6, hence, the value of 'n' is 1 which is a positive integer. So, it follows the fourth condition of aromaticity also.
4. But it does not have its π electrons in conjugation. Here, it does not meet the conditions of Huckel's aromaticity rule.

Based upon the above observation, as cycloheptatriene (option 1)  has not a complete conjugated structure or does not have all π electrons in conjugation, it does not obey Huckel's rule of aromaticity and hence is not aromatic.

Hence, the correct answer is option 1.