Which pair is identical ?

Options

A.
B.
C.
D.

Explanation

To determine which pair of compounds is identical, we will analyze each pair systematically based on their structural configurations and stereochemistry.

### Step-by-Step Solution:

1. **Identify the First Pair:**
- The first compound has two methyl groups attached to the same carbon.
- Since there are no chiral centers in this molecule, it is symmetrical.
- Thus, both structures are identical.

2. **Analyze the Second Pair:**
- The second compound has a hydroxyl group (OH), a bromine (Br), and an ethyl group (Et) attached to two different carbon centers.
- We need to determine the R/S configuration for both chiral centers.
- For the first carbon:
- Assign priorities: OH (1), Br (2), CH3 (3), H (4).
- The configuration is clockwise (R) when H is in the back, but since H is in front, we flip it to S.
- For the second carbon:
- Assign priorities: Br (1), OH (2), Et (3), H (4).
- The configuration is counterclockwise (S) when H is in the back, but since H is in front, we flip it to R.
- Since the configurations are different (R and S), these compounds are not identical.

3. **Examine the Third Pair:**
- The third compound has two hydroxyl groups and we need to analyze their configurations.
- If we rotate one of the molecules by 180 degrees, we find that the positions of the OH groups swap.
- This results in an enantiomer relationship, indicating that they are not identical.

4. **Evaluate the Fourth Pair:**
- The fourth compound also has two chiral centers.
- For the first carbon:
- Assign priorities: Br (1), Cl (2), CH3 (3), H (4).
- The configuration is counterclockwise (S).
- For the second carbon:
- Assign priorities: Br (1), Cl (2), CH3 (3), H (4).
- The configuration is clockwise (R).
- Since one is S and the other is R, these compounds are also not identical.

### Conclusion:
The only pair that is identical is the **first pair** of compounds. The other pairs are either enantiomers or different molecules.

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