Which of the following is not planar in structure?

Options

A. `CH_2=CH-CH=CH_2`
B. `CH_2=CH-CN`
C. `CH_2=C=CH_2`
D. `CH_2=C=C=CH_2`

Answer

To determine which of the given structures is not planar, we need to analyze the hybridization of the carbon atoms in each option. The key points to remember are:

1. **Hybridization Types**:
- **sp Hybridization**: Linear structure, bond angle of 180°.
- **sp² Hybridization**: Planar structure, bond angle of 120°.
- **sp³ Hybridization**: Non-planar (tetrahedral), bond angle of approximately 109.5°.

2. **Planarity Considerations**:
- Molecules with only sp² or sp hybridized carbons are generally planar.
- Molecules with sp³ hybridized carbons are non-planar.
- Even with sp² hybridized carbons, steric effects and pi-pi interactions can lead to non-planarity.

### Step-by-Step Solution:

1. **Identify the Hybridization of Each Carbon**:
- For each option, determine the hybridization of the carbon atoms based on the number of bonds (single, double, triple) they form.

2. **Evaluate Each Option**:
- **Option 1**: CH₃-CH₂-CH₃ (Propane)
- All carbons are sp³ hybridized (3 sp³ carbons).
- **Conclusion**: Non-planar.

- **Option 2**: CH₂=CH-CH=CH₂ (Butadiene)
- All carbons are sp² hybridized (4 sp² carbons).
- **Conclusion**: Planar.

- **Option 3**: CH₂=CH-CH=CH₂ (Allene)
- The first and last carbons are sp² hybridized, and the middle carbon is sp hybridized.
- Due to pi-pi interactions, the structure is non-planar.
- **Conclusion**: Non-planar.

- **Option 4**: CH₂=CH-CH₂ (Vinyl group)
- All carbons are sp² hybridized.
- **Conclusion**: Planar.

3. **Final Decision**:
- Among the options, the structure that is not planar is **Option 3** (Allene).

### Answer:
**Option 3** is not planar in structure.