Which of the following has highest `K_(a)` value?

Options

A.
B.
C.
D.

Answer

To determine which of the given options has the highest \( K_a \) value, we need to analyze the structure of the acids provided and their ability to stabilize the conjugate base formed after dissociation. Here’s a step-by-step solution:

### Step 1: Understanding \( K_a \)
The acid dissociation constant \( K_a \) is a measure of the strength of an acid in solution. It is defined as:
\[ K_a = \frac{[H^+][A^-]}{[HA]} \]
Where:
- \([H^+]\) is the concentration of hydrogen ions,
- \([A^-]\) is the concentration of the conjugate base,
- \([HA]\) is the concentration of the undissociated acid.

### Step 2: Relationship Between \( K_a \) and Acid Strength
The strength of an acid is directly proportional to the stability of its conjugate base. A more stable conjugate base will result in a higher \( K_a \) value.

### Step 3: Analyzing the Given Options
We need to analyze the effects of substituents on the stability of the conjugate base. The options involve a carboxylic acid with different substituents (like fluorine) at various positions (ortho, meta, para, and ipso).

1. **Ortho Position**: The fluorine atom is close to the carboxyl group and will exert an inductive effect, stabilizing the negative charge on the conjugate base.

2. **Meta Position**: The fluorine atom is further away from the carboxyl group, resulting in less stabilization of the negative charge.

3. **Para Position**: Similar to the meta position, the fluorine atom is not as effective in stabilizing the negative charge compared to the ortho position.

4. **Ipso Position**: The fluorine atom is directly attached to the carbon bearing the carboxyl group, providing maximum stabilization of the negative charge on the conjugate base.

### Step 4: Conclusion
The stability of the conjugate base is highest when the electronegative fluorine atom is in the ipso position, as it can exert the strongest inductive effect on the carboxylate ion formed after dissociation. Therefore, the acid with the fluorine atom at the ipso position will have the highest \( K_a \) value.

### Final Answer
The option with the highest \( K_a \) value is **Option D**.

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