Species in which all C-C bonds are not equal is/are

Options

A.
B.
C.
D.

Answer

To determine which species have unequal C-C bonds, we need to analyze each of the given options based on their structural characteristics and resonance behavior. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Concept of Resonance
- Resonance occurs in compounds where there are multiple valid Lewis structures (resonance structures) that contribute to the overall structure. In such cases, the bonds become equivalent due to delocalization of electrons.
- If a compound does not exhibit resonance, the C-C bonds may be unequal.

### Step 2: Analyze Each Option
1. **Option 1**:
- The structure has 4 pi electrons (cyclic and planar).
- It follows the 4n pi electron rule, indicating it is anti-aromatic.
- Anti-aromatic compounds do not exhibit resonance.
- **Conclusion**: C-C bonds are not equal. (This option is correct.)

2. **Option 2**:
- This structure is benzene, which has 6 pi electrons (cyclic, planar, and fully conjugated).
- It follows the 4n + 2 pi electron rule (Hückel's rule), making it aromatic.
- Aromatic compounds exhibit resonance.
- **Conclusion**: C-C bonds are equal. (This option is incorrect.)

3. **Option 3**:
- This structure has 10 pi electrons (cyclic but not planar due to steric repulsion).
- Although it follows the 4n + 2 pi electron rule, it is non-aromatic because it is not planar.
- Non-aromatic compounds do not exhibit resonance.
- **Conclusion**: C-C bonds are not equal. (This option is correct.)

4. **Option 4**:
- This structure has 8 pi electrons (cyclic and planar).
- It follows the 4n pi electron rule, indicating it is anti-aromatic.
- Anti-aromatic compounds do not exhibit resonance.
- **Conclusion**: C-C bonds are not equal. (This option is correct.)

### Final Answer
The species in which all C-C bonds are not equal are **Option 1, Option 3, and Option 4**.

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