Which of the following acids have the lowest `pK_(a)` value :-

Options

A. `CH_(3)overset(Cl)overset(|)CH-COOH`
B. `Cl-CH_(2)-CH_(2)-COOH`
C. `C Cl_(3)COOH`
D. `CHCl_(2)COOH`

Answer

To determine which of the given acids has the lowest pKa value, we need to analyze the acidity of each acid based on the presence of electron-withdrawing groups. The pKa value is inversely related to the strength of the acid; a lower pKa value indicates a stronger acid.

### Step-by-Step Solution:

1. **Understanding pKa and Acidity**:
- The pKa value is the negative logarithm of the acid dissociation constant (Ka). A lower pKa value indicates a stronger acid because it means that the acid dissociates more completely in solution, producing more H⁺ ions.

2. **Identify the Acids**:
- We have four acids to consider:
1. CH3CH(Cl)COOH (Chloroacetic acid)
2. CH2ClC(Cl)COOH (Dichloroacetic acid)
3. CCl3COOH (Trichloroacetic acid)
4. CHCl2C(Cl)COOH (Chloroacetic acid with two Cl groups)

3. **Analyze Electron-Withdrawing Effects**:
- Electron-withdrawing groups (EWGs) increase acidity by stabilizing the negative charge on the conjugate base after the acid donates a proton (H⁺).
- The strength of the EWG effect depends on the number and position of the electron-withdrawing groups relative to the carboxylic acid group.

4. **Comparing the Acids**:
- **Acid 1 (Chloroacetic acid)**: Has one Cl group, which exerts a moderate -I effect.
- **Acid 2 (Dichloroacetic acid)**: Has two Cl groups, which increases the -I effect, making it more acidic than acid 1.
- **Acid 3 (Trichloroacetic acid)**: Has three Cl groups, resulting in a very strong -I effect, making it the most acidic among the options.
- **Acid 4 (Chloroacetic acid with two Cl groups)**: Has two Cl groups but is less effective than acid 3 due to the presence of one less Cl.

5. **Conclusion**:
- The acid with the strongest electron-withdrawing effect (Trichloroacetic acid) will have the lowest pKa value. Therefore, the answer is **Trichloroacetic acid (CCl3COOH)**, which corresponds to option 3.

### Final Answer:
The acid with the lowest pKa value is **Trichloroacetic acid (CCl3COOH)**.

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