Which one is most stable ?

Options

A. Cyclopropane
B. Cyclobutane
C. Cyclopentane
D. Cyclohexane

Answer

To determine which of the given cycloalkanes (cyclopropane, cyclobutane, cyclopentane, or cyclohexane) is the most stable, we need to analyze the concept of ring strain and the structure of each cycloalkane.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **Cyclopropane**: A three-membered carbon ring (C3H6).
- **Cyclobutane**: A four-membered carbon ring (C4H8).
- **Cyclopentane**: A five-membered carbon ring (C5H10).
- **Cyclohexane**: A six-membered carbon ring (C6H12).

2. **Understand Ring Strain**:
- Ring strain occurs in cyclic compounds due to the deviation of bond angles from the ideal tetrahedral angle of 109.5 degrees.
- Smaller rings (like cyclopropane and cyclobutane) have significant ring strain because their bond angles are much less than 109.5 degrees.

3. **Analyze Each Cycloalkane**:
- **Cyclopropane**: The bond angles are approximately 60 degrees, leading to high ring strain and instability.
- **Cyclobutane**: The bond angles are about 90 degrees, which is still below the ideal angle, resulting in some ring strain but less than cyclopropane.
- **Cyclopentane**: The bond angles are closer to 108 degrees, which is near the ideal angle, so it has less ring strain compared to cyclopropane and cyclobutane but still some strain.
- **Cyclohexane**: The bond angles are approximately 109.5 degrees, which is ideal. Cyclohexane has no significant ring strain and its hydrogens are staggered, minimizing repulsion.

4. **Conclusion**:
- Among cyclopropane, cyclobutane, cyclopentane, and cyclohexane, cyclohexane is the most stable due to its ideal bond angles and absence of ring strain.

### Answer:
The most stable cycloalkane is **cyclohexane**.

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