Select the acid with the highest Ka (i.e., lowest `pK_a` )

Options

A.
B.
C.
D.

Explanation

To determine which acid has the highest \( K_a \) (and therefore the lowest \( pK_a \)), we need to analyze the acidic strength of the given compounds based on their ability to donate protons (H⁺) and the stability of the resulting anions.

### Step-by-Step Solution:

1. **Understanding \( K_a \) and \( pK_a \)**:
- \( K_a \) is the acid dissociation constant, which measures the strength of an acid in solution. A higher \( K_a \) value indicates a stronger acid.
- \( pK_a \) is the negative logarithm of \( K_a \): \( pK_a = -\log(K_a) \). Therefore, a lower \( pK_a \) value indicates a stronger acid.

2. **Analyzing the Compounds**:
- We will analyze the given compounds based on their ability to stabilize the anion formed after the acid donates a proton.
- The presence of electron-withdrawing groups (EWGs) increases the acidity by stabilizing the negative charge on the anion.

3. **Identifying the Compounds**:
- Let's denote the compounds as follows:
- A: \( CH_3COOH \) (acetic acid)
- B: \( CCl_3COOH \) (trichloroacetic acid)
- C: \( ICH_2COOH \) (iodoacetic acid)
- D: \( ClCH_2COOH \) (chloroacetic acid)

4. **Stability of Anions**:
- **Compound A**: The anion \( CH_3COO^- \) is destabilized by the electron-donating \( CH_3 \) group.
- **Compound B**: The anion \( CCl_3COO^- \) is stabilized by three \( Cl \) groups, which have a strong -I (inductive) effect.
- **Compound C**: The anion \( ICH_2COO^- \) is stabilized by the \( I \) group, but it is less effective than \( Cl \) due to its larger size and weaker -I effect.
- **Compound D**: The anion \( ClCH_2COO^- \) is stabilized by one \( Cl \) group, but not as effectively as in compound B.

5. **Ranking the Acids**:
- Based on the analysis:
- **B** (trichloroacetic acid) has the highest acidity due to the strong electron-withdrawing effect of three \( Cl \) atoms.
- **D** (chloroacetic acid) is next due to one \( Cl \) group.
- **A** (acetic acid) is less acidic because of the electron-donating \( CH_3 \) group.
- **C** (iodoacetic acid) is the least acidic due to the weaker -I effect of \( I \).

6. **Conclusion**:
- The acid with the highest \( K_a \) (and thus the lowest \( pK_a \)) is **B: \( CCl_3COOH \)** (trichloroacetic acid).

### Final Answer:
**B: \( CCl_3COOH \)** (trichloroacetic acid) has the highest \( K_a \) and the lowest \( pK_a \).