Which of the following will have the least hindered rotation about carbon-carbon bonds?

Options

A. Ethane
B. Ethylene
C. Acetylene
D. Hexachlorethane

Explanation

To determine which compound has the least hindered rotation about carbon-carbon bonds, we will analyze the given options based on their structures and the substituents attached to the carbon atoms.

### Step-by-Step Solution:

1. **Identify the Compounds**: The question provides us with four options, which include alkanes and possibly other compounds. We will focus on the two mentioned in the transcript: ethane (C2H6) and hexachloroethane (C2Cl6).

2. **Draw the Structures**:
- **Ethane (C2H6)**: Ethane consists of two carbon atoms connected by a single bond, with three hydrogen atoms attached to each carbon. The structure can be represented as:
```
H H
\ /
C-C
/ \
H H
```
- **Hexachloroethane (C2Cl6)**: This compound also consists of two carbon atoms connected by a single bond, but each carbon is attached to three chlorine atoms instead of hydrogen atoms. The structure can be represented as:
```
Cl Cl
\ /
C-C
/ \
Cl Cl
```

3. **Analyze Steric Hindrance**:
- In ethane, the hydrogen atoms are relatively small and allow for free rotation around the carbon-carbon bond without significant steric hindrance.
- In hexachloroethane, the chlorine atoms are larger and bulky, which creates steric hindrance. This bulkiness restricts the rotation around the carbon-carbon bond.

4. **Conclusion**: Since ethane has smaller substituents (hydrogens) compared to hexachloroethane (chlorines), it experiences less steric hindrance. Therefore, ethane will have the least hindered rotation about its carbon-carbon bonds.

5. **Final Answer**: The compound with the least hindered rotation about carbon-carbon bonds is **ethane**.