Question

Assertion (A): The compound cycloocta $-1,3,5,7-$ tetraene has the following structural formula


It is cyclic and has conjugated $8\pi -$ electron system but it is not an aromatic compound.

Reason $\left(R\right):(4n+2)\pi -$ electron rule dose not hold good and the ring is not planar.

• A. A is not correct but R is correct.
• B. Both A and R are correct but R is not the correct explanation of A.
• C. Both A and R are not correct.
• D. Both A and R are correct and R is the correct explanation of A.

Answer 1

The correct option is D Both A and R are correct and R is the correct explanation of A.

Solution:

Condition for Aromaticity-

For a molecule to be aromatic, it must be planar,cyclic,conjugated system, having $(4n+2)\pi$ electrons where $n=0,1,\mathrm{2..}etc.$

And for a molecule to be antiaromatic, it must be planar, cyclic, conjugated as well as have $4n\pi$ electrons where n = 0,1,2..etc.

Since cycloocta - 1,3,5,7-tetraene has a non- planar structure, thus it is not an aromatic compound.

Analsing the Given compound-

The structural formula of cycloocta- 1,3,5,7-tetraene is:


* This compound is cyclic
* It has conjugated $8\pi$ electron system
* there exists close conjugation

* Every member of the ring is $s{p}^2$ hybridized, but in order to escape from anti-aromaticity, and aviod torsional strain, cyclooctatetraene adopts a nonplanar geometry and thus attains this kind of tub-shape.

* It has a tub shape, i.e.,it is not planar

Hence,assertion and reason both are correct and reason is the correct explanation of assertion. So Option (A) is correct.