Question

Given below are two statements: one is labelled as Assertion A and, the other is labelled as Reason R.
Assertion A: [6] Annulene, [8] Annulene and cis-[10] Annulene, are respectively aromatic, not-aromatic and aromatic.
Reason R: Planarity is one, of the requirements of aromatic systems.
In the light of the above statements, choose the most appropriate answer from the options given below.

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

Answer 1

The Correct Option is D

To determine the correct answer, let's examine the given statements:

Assertion A:

[6] Annulene, [8] Annulene, and cis-[10] Annulene are respectively aromatic, not-aromatic, and aromatic.

Reason R:

Planarity is one of the requirements of aromatic systems.

Explanation:

1. Hückel's Rule: Aromatic compounds must obey Hückel's rule, having 4n + 2 π electrons (where n is an integer). Additionally, they must be planar and fully conjugated.
2. [6] Annulene:
   • It has 6 π electrons (n = 1). It is planar and fully conjugated, thus aromatic.
3. [8] Annulene:
   • It has 8 π electrons, which does not follow Hückel's rule (4n + 2 rule).
   • Additionally, it cannot maintain planarity due to steric hindrance among hydrogen atoms inside the ring.
   • Therefore, it is non-aromatic.
4. cis-[10] Annulene:
   • It has 10 π electrons, complying with Hückel's rule (n = 2).
   • However, due to steric strain, it is difficult for cis-[10] annulene to remain planar.
   • Therefore, the assertion that it is aromatic is incorrect.

Thus, Assertion A has incorrect information about the aromatic nature of cis-[10] Annulene, making it incorrect.

Reason R correctly states a requirement for aromaticity: planarity. This is a true statement and a necessary condition for aromaticity. Therefore, Reason R is correct.

Conclusion:

Based on the above analysis, Assertion A is not correct but Reason R is correct. Hence, the correct answer is:

A is not correct but R is correct.