Which of the following conformers for ethylene glycol is most stable?

Options

A.
B.
C.
D.

Answer

To determine which conformer of ethylene glycol is most stable, we will analyze each conformer based on their spatial arrangements and the interactions between the hydroxyl (OH) groups.

### Step-by-Step Solution:

1. **Identify the Conformers**:
- The conformers of ethylene glycol can be categorized as fully eclipsed, eclipsed, gauche, and anti.

2. **Analyze the Fully Eclipsed Conformer**:
- In the fully eclipsed conformer, the two OH groups are directly behind each other, resulting in a dihedral angle of 0 degrees.
- **Stability**: This arrangement is highly unstable due to significant steric repulsion between the identical OH groups.

3. **Analyze the Eclipsed Conformer**:
- In the eclipsed conformer, the OH groups are also aligned, but there is a hydrogen atom behind one of the OH groups, leading to a dihedral angle of 0 degrees between the OH and H.
- **Stability**: This conformer is also unstable due to the eclipsing interactions, although slightly less so than the fully eclipsed conformer.

4. **Analyze the Gauche Conformer**:
- In the gauche conformer, the dihedral angle between the two OH groups is 60 degrees.
- **Stability**: This arrangement allows for potential hydrogen bonding between the two OH groups, which introduces stabilizing intermolecular forces. This makes the gauche conformer more stable than the previously mentioned conformers.

5. **Analyze the Anti Conformer**:
- In the anti conformer, the two OH groups are positioned opposite each other, minimizing steric repulsion.
- **Stability**: While this conformer is relatively stable due to reduced repulsion, it lacks the stabilizing hydrogen bonding present in the gauche conformer.

6. **Conclusion**:
- After analyzing all conformers, the **gauche conformer** (option C) is the most stable due to the presence of hydrogen bonding between the OH groups, which provides additional stabilization compared to the other conformers.

### Final Answer:
The most stable conformer of ethylene glycol is the **gauche conformer**.