Which of the following aromatic acids is most acidic?

Options

A.
B.
C.
D.

Answer

To determine which of the following aromatic acids is most acidic, we will analyze each option based on the effects of substituents on acidity.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the four aromatic acids as follows:
- Option 1: Benzoic acid (C6H5COOH)
- Option 2: Ortho-hydroxybenzoic acid (C6H4(OH)COOH)
- Option 3: Meta-hydroxybenzoic acid (C6H4(OH)COOH)
- Option 4: Para-hydroxybenzoic acid (C6H4(OH)COOH)

2. **Understanding Acidity**:
Acidity in carboxylic acids is determined by their ability to donate a proton (H+). The stability of the resulting carboxylate anion (RCOO-) after deprotonation is crucial. More stable anions correspond to stronger acids.

3. **Analyze Benzoic Acid (Option 1)**:
- Benzoic acid can donate H+ to form a carboxylate anion (C6H5COO-).
- The negative charge on the carboxylate anion is stabilized by resonance, making benzoic acid moderately acidic.

4. **Analyze Ortho-Hydroxybenzoic Acid (Option 2)**:
- In ortho-hydroxybenzoic acid, the OH group is at the ortho position.
- The ortho effect enhances acidity because the OH group can stabilize the carboxylate anion through intramolecular hydrogen bonding and resonance.
- This makes ortho-hydroxybenzoic acid the strongest acid among the options.

5. **Analyze Meta-Hydroxybenzoic Acid (Option 3)**:
- In meta-hydroxybenzoic acid, the OH group is at the meta position.
- The OH group exerts a -I (inductive) effect, which increases acidity slightly, but lacks the resonance stabilization seen in ortho substitution.
- Thus, it is more acidic than benzoic acid but less acidic than ortho-hydroxybenzoic acid.

6. **Analyze Para-Hydroxybenzoic Acid (Option 4)**:
- In para-hydroxybenzoic acid, the OH group is at the para position.
- The OH group exerts both +R (resonance) and -I (inductive) effects.
- However, the +R effect dominates, which decreases the acidity compared to benzoic acid.

7. **Conclusion**:
- Based on the analysis, the order of acidity is:
- Ortho-hydroxybenzoic acid > Meta-hydroxybenzoic acid > Benzoic acid > Para-hydroxybenzoic acid.
- Therefore, the most acidic aromatic acid among the options is **Option 2: Ortho-hydroxybenzoic acid**.

### Final Answer:
The most acidic aromatic acid is **Option 2: Ortho-hydroxybenzoic acid**.