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

Options

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

Answer

To determine the best representation of (S)-2-fluorobutane, we will follow these steps:

### Step 1: Identify the Structure of 2-Fluorobutane
2-Fluorobutane is a four-carbon chain (butane) with a fluorine atom attached to the second carbon. The structure can be represented as follows:

```
CH3
|
CH3-CH-CH2-CH3
|
F
```

### Step 2: Assign Priorities Using Cahn-Ingold-Prelog (CIP) Rules
According to the CIP rules, we need to assign priorities to the substituents on the chiral carbon (C2 in this case). The priorities are assigned based on the atomic number of the atoms directly attached to the chiral center:

1. **Fluorine (F)** - Atomic number 9 (highest priority)
2. **Methyl group (CH3)** - Carbon has atomic number 6 (second priority)
3. **Ethyl group (CH2-CH3)** - Carbon has atomic number 6, but it is connected to more carbons (third priority)
4. **Hydrogen (H)** - Atomic number 1 (lowest priority)

### Step 3: Determine the Configuration
To determine the configuration (R or S), we need to visualize the molecule in 3D. The lowest priority group (H) should be placed at the back (dashed).

1. Arrange the groups so that the lowest priority (H) is at the back.
2. Trace the path from the highest priority (1) to the second (2) to the third (3).

In this case, if we trace from F (1) to CH3 (2) to CH2-CH3 (3), we will see that the path is counterclockwise.

### Step 4: Assign the Configuration
Since the path traced is counterclockwise and the lowest priority group is at the back, the configuration is S. However, if we were to swap the position of the lowest priority group (H) with any other group, the configuration would change.

### Step 5: Conclusion
The best representation of (S)-2-fluorobutane would be the one that accurately depicts the S configuration with the fluorine at the second carbon and the hydrogen at the back.

### Final Structure
The correct structure of (S)-2-fluorobutane can be represented as follows:

```
CH3
|
CH3-CH-CH2-CH3
|
F
```
Where H is depicted as being behind the plane of the paper.