AIIMS 2005 Chemistry Stability of Anions Assertion Reason Question

To analyze the given question regarding the stability of the cyclopentadienyl anion compared to the allyl anion, we need to delve into the concepts of aromaticity and the structural characteristics of these anions.

Assertion:

Cyclopentadienyl anion is much more stable than allyl anion.

Reason:

Cyclopentadienyl anion is aromatic in character.

Explanation:

1. Understanding the Cyclopentadienyl Anion:

   • The cyclopentadienyl anion, $\text{C}_5\text{H}_5^-$, is derived from cyclopentadiene by removing a proton. Its structure consists of 5 carbon atoms in a cyclic arrangement with a total of 6 π electrons.
   • According to Hückel's rule, for a compound to be aromatic, it must be cyclic, planar, and have $4n + 2$ π electrons, where $n$ is a non-negative integer. For the cyclopentadienyl anion, $n = 1$ (since it has 6 π electrons), fulfilling the criteria for aromaticity.
   • This aromatic character imparts significant stability to the cyclopentadienyl anion due to resonance, where the negative charge is delocalized across the entire ring structure.

2. Understanding the Allyl Anion:

   • The allyl anion, $\text{C}_3\text{H}_5^-$, has a linear structure with 3 carbon atoms. It has 4 π electrons involved in the resonance, which also contributes to its stability.
   • However, the allyl anion does not possess aromaticity as it lacks a cyclic structure. While it is stabilized by resonance, it does not achieve the same level of stability as the cyclopentadienyl anion.

3. Comparison of Stability:

   • The greater stability of the cyclopentadienyl anion, in comparison to the allyl anion, can be attributed to its aromaticity, which allows for extensive delocalization of electrons, leading to lower energy and greater stability.

Conclusion:

• Both the assertion and reason are true: the cyclopentadienyl anion is indeed more stable than the allyl anion due to its aromatic character. Thus, the correct answer is:

A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Clarifying Other Options:

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion: This option is incorrect because the reason directly explains why the assertion is true (the stability due to aromaticity).
• C) Assertion is true but Reason is false: This option is incorrect because both the assertion and reason are true.
• D) Both Assertion and Reason are false: This option is incorrect as both statements are true.

Summary:

The cyclopentadienyl anion’s stability is significantly higher than that of the allyl anion due to its aromatic nature, which allows for greater electron delocalization and resonance stabilization, confirming that the correct answer is A.