Which of the following is not an isomer of 3-methylbut-1-yne ?

Options

A. Pent-1-yne
B. Buta-1,3-diene
C. Pent-2-yne
D. Penta-1,3-diene

Explanation

To determine which of the following compounds is not an isomer of 3-methylbut-1-yne, we first need to understand what isomers are and analyze the given compounds based on their chemical formula.

### Step-by-Step Solution:

1. **Identify the Structure of 3-Methylbut-1-yne:**
- The name "3-methylbut-1-yne" indicates that there are 5 carbon atoms (but) and a triple bond (yne) at the first carbon.
- The structure can be represented as:
```
CH3-C≡C-CH2-CH3
```
- The molecular formula for 3-methylbut-1-yne is C5H8.

2. **Count Carbons and Hydrogens:**
- From the structure, we can see there are 5 carbon atoms and 8 hydrogen atoms.
- Therefore, the molecular formula is confirmed as C5H8.

3. **Analyze Each Option:**
- We need to check each given compound to see if it has the same molecular formula (C5H8) and if it can be structured differently (isomer).

- **Option 1: Pent-1-yne**
- Structure: CH3-C≡C-CH2-CH3
- This has 5 carbons and 8 hydrogens (C5H8). It is an isomer.

- **Option 2: Buta-1,3-diene**
- Structure: CH2=CH-CH=CH2
- This has 4 carbons and 6 hydrogens (C4H6). It is not an isomer of C5H8.

- **Option 3: Pent-2-ene**
- Structure: CH3-CH=C-CH2-CH3
- This has 5 carbons and 8 hydrogens (C5H8). It is an isomer.

- **Option 4: Pent-1,3-diene**
- Structure: CH2=C=CH-CH2-CH3
- This has 5 carbons and 8 hydrogens (C5H8). It is an isomer.

4. **Conclusion:**
- The compound that is not an isomer of 3-methylbut-1-yne is **Option 2: Buta-1,3-diene** (C4H6).

### Final Answer:
**Option 2: Buta-1,3-diene is not an isomer of 3-methylbut-1-yne.**