Which one of the following is the most stable conformer?

Options

A.
B.
C.
D.

Explanation

To determine which conformer is the most stable among the given options (A, B, C, and D), we need to analyze the interactions present in each conformer. The stability of a conformer is influenced by the presence of attractive and repulsive interactions between the substituents on the carbon atoms.

### Step-by-Step Solution:

1. **Analyze Conformer A:**
- Structure: CH3 - H - OH (front carbon) and OH - CH3 (back carbon).
- Interactions: The two CH3 groups are 180 degrees apart, and the two OH groups are also 180 degrees apart.
- Conclusion: There are no steric hindrance or repulsive interactions. This conformer is relatively stable due to the absence of interactions.

2. **Analyze Conformer B:**
- Structure: CH3 - H (front carbon) and H - CH3 - OH (back carbon).
- Interactions: There is one gauche interaction (repulsive) between CH3 and OH, but there is also the potential for hydrogen bonding between the OH groups.
- Conclusion: The attractive hydrogen bonding partially offsets the repulsive interaction, making this conformer less stable than A.

3. **Analyze Conformer C:**
- Structure: CH3 - OH - H (front carbon) and CH3 - OH - H (back carbon).
- Interactions: There is one hydrogen bonding interaction (attractive) and no gauche interactions.
- Conclusion: The presence of hydrogen bonding without any repulsive interactions makes this conformer more stable than A and B.

4. **Analyze Conformer D:**
- Structure: OH - CH3 - OH - CH3 - H.
- Interactions: There is one gauche interaction (repulsive) present.
- Conclusion: The presence of a repulsive interaction makes this conformer less stable.

5. **Comparison of Stability:**
- Conformer A: No interactions (stable).
- Conformer B: One repulsive and one attractive interaction (less stable).
- Conformer C: One attractive interaction and no repulsive interactions (most stable).
- Conformer D: One repulsive interaction (least stable).

### Final Conclusion:
The conformer with the highest amount of attractive interactions and the lowest amount of repulsive interactions is Conformer C. Therefore, the most stable conformer is **option C**.

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