Which of the following is most stable confirmational isomer `:-`

Options

A.
B.
C.
D.

Answer

To determine the most stable conformational isomer of ethylene glycol (CH2OH-CH2OH), we need to analyze the different conformations that can arise from the rotation around the carbon-carbon single bond. Here’s a step-by-step solution:

### Step 1: Identify the Structure of Ethylene Glycol
Ethylene glycol has the following structure:
```
HO-CH2-CH2-OH
```
This means there are two hydroxyl (–OH) groups attached to the carbon atoms.

### Step 2: Understand Conformational Isomers
Conformational isomers are different arrangements of atoms that can be converted into one another by rotation around single bonds. For ethylene glycol, we can have several conformations, including:
- Staggered (Gauche)
- Eclipsed
- Anti

### Step 3: Analyze the Staggered (Gauche) Conformation
In the Gauche conformation, the two –OH groups are positioned at an angle that allows for some interaction between them. This conformation is generally more stable due to minimized steric hindrance compared to the eclipsed conformation.

### Step 4: Analyze the Anti Conformation
In the Anti conformation, the two –OH groups are positioned 180 degrees apart. While this arrangement minimizes steric repulsion, it does not allow for the potential stabilizing interactions that can occur between the hydroxyl groups.

### Step 5: Consider Intermolecular Hydrogen Bonding
The presence of hydroxyl groups allows for hydrogen bonding. In the Gauche conformation, the –OH groups can form intermolecular hydrogen bonds, which significantly increases stability. This interaction is not possible in the Anti conformation.

### Step 6: Conclusion
Considering the factors above, the Gauche conformation of ethylene glycol is the most stable due to the additional stability provided by intermolecular hydrogen bonding, despite the Anti conformation being favorable in terms of steric hindrance.

### Answer
The most stable conformational isomer of ethylene glycol is the Gauche conformation (Option 1).

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