The structure of (S)-2-fluorobutane is best represented by:

Options

A. `CH_(3) underset(F) underset(|)(C)HCH_(2)CH_(3)`
B.
C.
D.

Explanation

To determine the structure of (S)-2-fluorobutane from the given options, we will follow these steps:

### Step 1: Understand the Structure of 2-Fluorobutane
2-Fluorobutane is a four-carbon chain with a fluorine atom attached to the second carbon. The molecular formula is C4H9F. The stereogenic center is at the second carbon, which means it can exist in two enantiomeric forms: (R) and (S).

### Step 2: Identify the Chiral Center
The second carbon (C-2) is the chiral center because it is attached to four different groups:
1. Fluorine (F)
2. Ethyl group (C2H5)
3. Methyl group (CH3)
4. Hydrogen (H)

### Step 3: Assign Priorities to the Groups
Using the Cahn-Ingold-Prelog priority rules, we assign priorities based on atomic number:
1. Fluorine (F) - highest priority (1)
2. Ethyl group (C2H5) - second priority (2)
3. Methyl group (CH3) - third priority (3)
4. Hydrogen (H) - lowest priority (4)

### Step 4: Determine the Configuration
To determine whether the configuration is (R) or (S), we need to visualize the molecule in a way that places the lowest priority group (H) in the back (dashed).

### Step 5: Visualize the Molecule
In the wedge-dash representation:
- The fluorine is at the top (1)
- The ethyl group is on the right (2)
- The methyl group is on the left (3)
- The hydrogen is in the back (dashed)

### Step 6: Analyze the Arrangement
Now, if we look at the arrangement from the perspective of the highest priority (F) to the lowest priority (H), we can see the order:
1 (F) → 2 (C2H5) → 3 (CH3)

This arrangement goes in a counterclockwise direction, which indicates that the configuration is (S).

### Step 7: Check the Given Options
We need to check the options provided:
- **Option 1**: Not in wedge-dash form (cannot determine configuration)
- **Option 2**: Wedge-dash form but does not have H in the back
- **Option 3**: Correctly represents (S)-2-fluorobutane
- **Option 4**: Incorrectly represents the configuration

### Conclusion
The structure of (S)-2-fluorobutane is best represented by **Option 3**.

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