Which of the following species is least stable ?

Options

A.
B.
C.
D.

Explanation

To determine which of the given cyclic species is the least stable, we can apply Huckel's rule. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the species
We have four cyclic species to analyze. Let's denote them as A, B, C, and D.

### Step 2: Check the conditions of Huckel's rule
Huckel's rule states that for a compound to be aromatic, it must satisfy the following conditions:
1. The compound must be cyclic.
2. The compound must be planar.
3. The compound must be conjugated.
4. The compound must have a number of π electrons that fits the formula \(4n + 2\), where \(n\) is a non-negative integer.

### Step 3: Analyze each species

#### Species A:
- **Cyclic**: Yes (5-membered ring)
- **Planar**: Yes
- **Conjugated**: Yes (delocalization of π electrons)
- **π Electrons**: Total of 6 π electrons (2 from each double bond).
- \(n = 1\) (since \(4n + 2 = 6\))
- **Conclusion**: Aromatic and stable.

#### Species B:
- **Cyclic**: Yes (4-membered ring)
- **Planar**: Yes
- **Conjugated**: Yes
- **π Electrons**: Total of 6 π electrons.
- \(n = 1\) (since \(4n + 2 = 6\))
- **Conclusion**: Aromatic and stable.

#### Species C:
- **Cyclic**: Yes (3-membered ring)
- **Planar**: Yes
- **Conjugated**: Yes
- **π Electrons**: Total of 4 π electrons.
- \(n = 0\) (since \(4n + 2 = 4\))
- **Conclusion**: Anti-aromatic (as \(n\) must be an integer and here it is not valid). This species is least stable.

#### Species D:
- **Cyclic**: Yes (8-membered ring)
- **Planar**: Yes
- **Conjugated**: Yes
- **π Electrons**: Total of 10 π electrons.
- \(n = 2\) (since \(4n + 2 = 10\))
- **Conclusion**: Aromatic and stable.

### Step 4: Compare stability
- Aromatic compounds (A, B, D) are more stable than non-aromatic compounds.
- Anti-aromatic compounds (C) are the least stable.

### Final Conclusion
Among the four species, **Species C** is the least stable because it is anti-aromatic.

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