Question

Explain why the following systems are not aromatic?

Answer 1

Explain why the following systems are not aromatic:

For a compound to be aromatic, it must satisfy all of the following conditions:

• It must be cyclic
• It must be planar
• It must be fully conjugated
• It must contain (4n + 2) π electrons (Hückel’s rule)

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(i) Benzene ring with an exocyclic =CH2 group

In this structure, the double bond attached outside the ring (exocyclic =CH2) does not take part in cyclic conjugation. The π-electrons of this double bond are localized and are not delocalized over the ring.

Therefore, the system does not form a continuous cyclic conjugated loop.

Hence, this system is not aromatic.

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(ii) Cyclopentadiene

Cyclopentadiene contains two double bonds, but one carbon atom in the ring is sp3 hybridized. This breaks the continuity of p-orbital overlap required for complete conjugation.

Since the ring is not fully conjugated, it does not satisfy the basic requirement for aromaticity.

Therefore, cyclopentadiene is not aromatic.

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(iii) Cyclooctatetraene

Cyclooctatetraene contains eight π-electrons, which corresponds to a 4n system. If it were planar, it would be antiaromatic.

To avoid antiaromatic instability, cyclooctatetraene adopts a non-planar (tub-shaped) structure. Due to non-planarity, effective overlap of p-orbitals is not possible.

Hence, cyclooctatetraene is not aromatic. 

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Final Conclusion:

• (i) Not aromatic due to lack of cyclic conjugation
• (ii) Not aromatic due to incomplete conjugation
• (iii) Not aromatic due to non-planarity and 4n π electrons