Which one of the following compounds is not planar?

Options

A. `CH_2 = C = CH_2`
B. `CH_2= CH_2`
C. `CH _2= CH - CH = CH_2`
D. `CH_2 = CH – C H = _ CH`

Answer

To determine which compound is not planar among the given options, we need to analyze the hybridization and geometry of each compound. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Compounds
We have four compounds to analyze. Let's denote them as:
1. Compound 1 (Allene)
2. Compound 2 (Ethene)
3. Compound 3 (A different alkene)
4. Compound 4 (A compound with a triple bond)

### Step 2: Analyze Compound 1 (Allene)
- **Structure**: The structure of allene can be represented as H2C=C=CH2.
- **Hybridization**:
- The first carbon (C1) has three sigma bonds (to two hydrogens and one carbon), so it is **sp² hybridized**.
- The central carbon (C2) has two pi bonds and one sigma bond, making it **sp hybridized**.
- The third carbon (C3) is similar to C1 and is also **sp² hybridized**.
- **Geometry**: The presence of the sp hybridized carbon causes the molecule to have a non-planar arrangement due to steric hindrance from the pi bonds.

### Step 3: Analyze Compound 2 (Ethene)
- **Structure**: Ethene is represented as H2C=CH2.
- **Hybridization**:
- Both carbons are sp² hybridized since each carbon has three sigma bonds.
- **Geometry**: Ethene is planar due to the sp² hybridization.

### Step 4: Analyze Compound 3 (Another Alkene)
- **Structure**: Assume a structure like H2C=CH-CH2-CH=CH2.
- **Hybridization**:
- All carbons are sp² hybridized as they each have three sigma bonds.
- **Geometry**: This compound is also planar due to sp² hybridization.

### Step 5: Analyze Compound 4 (Triple Bond Compound)
- **Structure**: A compound like H2C≡C-CH2.
- **Hybridization**:
- The terminal carbons (C1 and C3) are sp² hybridized (three sigma bonds).
- The central carbon (C2) is sp hybridized (one sigma and two pi bonds).
- **Geometry**: The presence of the sp hybridized carbon does not disrupt the overall planarity of the molecule since it is attached to sp² hybridized carbons.

### Conclusion
Among the four compounds analyzed:
- **Compound 1 (Allene)** is the only compound that is **not planar** due to the presence of the sp hybridized carbon and the steric hindrance caused by the pi bonds.

### Final Answer
**The compound that is not planar is Compound 1 (Allene).**

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