Which of the following will have highest value of `pk_a` ?

Options

A. `FCH_22 CH_2 COOH`
B. `CH_3CH.F.COOH`
C. `CH_3CH.Br.COOH`
D. `CH_3CH_2COOH`

Explanation

To determine which of the given compounds has the highest value of pKa, we need to understand the relationship between pKa and acidity. The pKa value is inversely related to the strength of an acid; a stronger acid has a lower pKa value, while a weaker acid has a higher pKa value. Therefore, to find the compound with the highest pKa, we need to identify the weakest acid among the options provided.

### Step-by-Step Solution:

1. **Identify the Compounds**: The question refers to several compounds, which are likely variations of hydrocarbons with different substituents (like fluorine, bromine, etc.). We need to analyze these compounds based on their acidic strength.

2. **Understand Acidic Strength**: The acidic strength of a compound is influenced by the stability of its conjugate base. If the conjugate base is more stable, the acid is stronger, and thus has a lower pKa. Conversely, if the conjugate base is less stable, the acid is weaker, leading to a higher pKa.

3. **Analyze the Substituents**:
- In the first case, we have a compound with a fluorine substituent (fCH2).
- In the second case, we have a compound with a methyl group and a fluorine substituent (CH3, CH).
- In the third case, we have a compound with a bromine substituent (Br).
- In the fourth case, we have a compound with no strong electron-withdrawing groups (CH3, CH2).

4. **Rank the Acids**:
- The compound with fluorine (fCH2) is likely to be the most acidic due to the strong electronegativity of fluorine, which stabilizes the conjugate base.
- The compound with bromine (Br) is less acidic than the one with fluorine but more acidic than those without strong electron-withdrawing groups.
- The compound with no strong electron-withdrawing groups (CH3, CH2) will be the least acidic.

5. **Conclusion**: Since the compound with no strong electron-withdrawing groups is the least acidic, it will have the highest pKa value. Therefore, the answer is the fourth option.

### Final Answer:
The compound with the highest value of pKa is the one with no strong electron-withdrawing groups (option 4).

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