Which will be the correct stability order of the different conformations of n - butane ?

Options

A. Fully - eclipsed `gt` eclipsed `gt` gauche `gt` anti - staggered
B. Anti - staggered `gt` eclipsed `gt` gauche `gt` fully - eclipsed
C. Anti - staggered `gt`gauche `gt` eclipsed `gt` fully - eclipsed
D. Gauche `gt` anti - staggered `gt` eclipsed `gt` fully - eclipsed

Answer

To determine the correct stability order of the different conformations of n-butane, we need to analyze the various conformations that arise from the rotation around the carbon-carbon single bond. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Conformations of n-butane
n-butane has several conformations due to the rotation around the C-C single bond. The main conformations are:
1. **Fully Eclipsed**
2. **Partially Eclipsed**
3. **Gauche (Staggered)**
4. **Anti (Staggered)**

### Step 2: Analyze Each Conformation
- **Fully Eclipsed**: In this conformation, the two methyl groups (CH3) are directly aligned with each other. This arrangement leads to maximum steric hindrance and torsional strain, making it the least stable conformation.

- **Partially Eclipsed**: In this conformation, one methyl group is eclipsed with a hydrogen atom. This arrangement is more stable than the fully eclipsed conformation but still has some steric hindrance.

- **Gauche (Staggered)**: In this conformation, the two methyl groups are staggered but are 60 degrees apart. This conformation is more stable than the eclipsed forms due to reduced steric hindrance.

- **Anti (Staggered)**: In this conformation, the two methyl groups are opposite each other (180 degrees apart). This arrangement minimizes steric hindrance and torsional strain, making it the most stable conformation.

### Step 3: Establish Stability Order
Based on the analysis:
1. **Anti (Staggered)** - Most stable
2. **Gauche (Staggered)**
3. **Partially Eclipsed**
4. **Fully Eclipsed** - Least stable

Thus, the correct stability order of the different conformations of n-butane is:
**Anti > Gauche > Partially Eclipsed > Fully Eclipsed**

### Step 4: Verify Options
Now, we can verify the options provided in the question:
- The correct order is **Anti > Gauche > Partially Eclipsed > Fully Eclipsed**.

### Conclusion
The correct answer is the option that reflects this stability order.

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