Which of the following pairs of compounds are not isomers?

Options

A. Propan-1-ol and methoxyethane
B. Propene and cyclopropene
C. propyne and propadiene
D. propyne and cyclopropene

Explanation

To determine which pairs of compounds are not isomers, we need to analyze the given options based on their structures and molecular formulas. Isomers are compounds that have the same molecular formula but different structural formulas.

### Step-by-Step Solution:

1. **Analyze Option A: Propanol and Methoxyethane**
- **Propanol Structure:** CH3-CH2-CH2-OH
- **Methoxyethane Structure:** CH3-CH2-O-CH3
- **Molecular Formula:** Both have the formula C3H8O.
- **Type:** These are functional isomers (alcohol vs ether).
- **Conclusion:** They are isomers.

2. **Analyze Option B: Propene and Cyclopropene**
- **Propene Structure:** CH3-CH=CH2
- **Cyclopropene Structure:** A three-membered ring with a double bond (C3H4).
- **Degree of Unsaturation:**
- Propene has a degree of unsaturation of 1 (one double bond).
- Cyclopropene has a degree of unsaturation of 2 (one double bond and one ring).
- **Conclusion:** They are not isomers because their degrees of unsaturation differ.

3. **Analyze Option C: Propene and Propadiene**
- **Propene Structure:** CH3-CH=CH2
- **Propadiene Structure:** CH2=C=CH2
- **Molecular Formula:** Both have the formula C3H4.
- **Type:** These are functional isomers (alkene vs diene).
- **Conclusion:** They are isomers.

4. **Analyze Option D: Propene and Cyclopropene (re-evaluation)**
- **Propene Structure:** CH3-CH=CH2
- **Cyclopropene Structure:** C3H4 (three-membered ring with a double bond).
- **Molecular Formula:** Different degrees of unsaturation as previously analyzed.
- **Conclusion:** They are not isomers.

### Final Answer:
The pair of compounds that are **not isomers** is **Option B: Propene and Cyclopropene**.