Which one 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 given options is not an isomer of 3-methylbut-1-yne, we first need to understand what isomers are. Isomers are compounds that have the same molecular formula but different structural formulas or arrangements of atoms.

### Step-by-Step Solution:

1. **Identify the Molecular Formula of 3-methylbut-1-yne**:
- The structure of 3-methylbut-1-yne consists of 5 carbon atoms and 8 hydrogen atoms.
- The molecular formula is C5H8.

2. **List the Options**:
- We need to evaluate the following options to see if they have the same molecular formula (C5H8):
- Option A: Pent-1-yne
- Option B: But-1,3-diene
- Option C: Pent-2-yne
- Option D: Penta-1,3-diene

3. **Check Each Option**:
- **Option A: Pent-1-yne**:
- Structure: C5H8 (5 carbons, 8 hydrogens)
- This is an isomer of 3-methylbut-1-yne.

- **Option B: But-1,3-diene**:
- Structure: C4H6 (4 carbons, 6 hydrogens)
- This does not match the molecular formula C5H8, so it is NOT an isomer.

- **Option C: Pent-2-yne**:
- Structure: C5H8 (5 carbons, 8 hydrogens)
- This is an isomer of 3-methylbut-1-yne.

- **Option D: Penta-1,3-diene**:
- Structure: C5H8 (5 carbons, 8 hydrogens)
- This is also an isomer of 3-methylbut-1-yne.

4. **Conclusion**:
- The only option that does not have the same molecular formula (C5H8) is **Option B: But-1,3-diene**. Therefore, it is not an isomer of 3-methylbut-1-yne.

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