Which of the following structures has the `S-`configuration at the chiral centre?

Options

A.
B.
C.
D.

Explanation

To determine which of the given structures has the `S-`configuration at the chiral center, we will follow these steps:

### Step 1: Identify the Chiral Center
First, we need to identify the chiral centers in each of the four structures provided. A chiral center is typically a carbon atom that is bonded to four different substituents.

### Step 2: Convert to Fischer Projection
For each structure, convert it into Fischer projection. In Fischer projections, the vertical lines represent bonds that are going away from the viewer, while the horizontal lines represent bonds that are coming towards the viewer.

### Step 3: Assign Priorities
Using the Cahn-Ingold-Prelog priority rules, assign priorities to the substituents attached to the chiral carbon:
- The atom with the highest atomic number gets the highest priority (1).
- If two atoms are the same, look at the next atoms in the substituent chain until a difference is found.
- The lowest priority substituent (4) should ideally be placed in a vertical position.

### Step 4: Determine Configuration
Once the priorities are assigned:
- If the sequence from highest to lowest priority (1 → 2 → 3) is clockwise, the configuration is `R`.
- If the sequence is counterclockwise, the configuration is `S`.
- Ensure that the lowest priority group (4) is in the vertical position while determining the configuration.

### Step 5: Analyze Each Structure
1. **First Structure**: Assign priorities and determine that it has an `R` configuration.
2. **Second Structure**: Assign priorities and determine that it has an `S` configuration.
3. **Third Structure**: Assign priorities and determine that it has an `R` configuration.
4. **Fourth Structure**: Assign priorities and determine that it has an `R` configuration.

### Conclusion
After analyzing all four structures, the one with the `S` configuration is the **Second Structure**.

### Final Answer
The structure with the `S-`configuration at the chiral center is **Option B**.

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