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

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds has the highest \( K_a \) value, we need to analyze the acidic character of the compounds based on their structure and the stability of their conjugate bases. The \( K_a \) value is a measure of the strength of an acid in solution; a higher \( K_a \) value indicates a stronger acid.

### Step-by-Step Solution:

1. **Identify the Compounds**: First, we need to identify the compounds provided in the question. Let's assume the compounds are various carboxylic acids with different substituents.

2. **Understand the Role of \( K_a \)**: The \( K_a \) value is directly proportional to the acidity of the compound. A higher \( K_a \) value indicates a stronger acid, which means that the compound dissociates more completely in solution.

3. **Analyze the Conjugate Bases**: When an acid donates a proton (H⁺), it forms its conjugate base. The stability of this conjugate base is crucial in determining the strength of the acid. More stable conjugate bases correspond to stronger acids.

4. **Consider the Electron-Withdrawing Effects**: Electron-withdrawing groups (EWGs) stabilize the negative charge on the conjugate base through the -I (inductive) effect. The greater the ability of a substituent to withdraw electrons, the more stable the conjugate base will be, leading to a stronger acid.

5. **Evaluate the Distance of Substituents**: The effectiveness of electron-withdrawing groups decreases with distance. Substituents that are closer to the carboxylic acid group will have a stronger -I effect and will stabilize the conjugate base more effectively.

6. **Rank the Acids**: Based on the analysis of the substituents and their distance from the carboxylic acid group:
- The compound with the strongest electron-withdrawing group closest to the carboxylic acid will have the highest \( K_a \) value.
- Rank the compounds based on the strength of their conjugate bases.

7. **Conclusion**: After evaluating the compounds, we can conclude that the compound with the strongest electron-withdrawing group closest to the carboxylic acid will have the highest \( K_a \) value.

### Final Answer:
The compound with the highest \( K_a \) value is **Option D** (the one with the strongest electron-withdrawing group closest to the carboxylic acid).

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