What will be the correct decreasing order of acid strength of the hydroxybenzoic acids? (Symbols and notations carry their usual meanings)

Options

A. `p - Hydr"ox"ybenzoic acid gt benzoic acid gt m - hydr"ox"ybenzoic acid gt o - hydr"ox"ybenzoic acid`
B. `o - Hydr"ox"ybenzoic acid gt m - hydr"ox"ybenzoic acid gt benzoic acid gt p - hydr"ox"ybenzoic acid`
C. `o - Hydr"ox"ybenzoic acid gt benzoic acid gt m - hydr"ox"ybenzoic acid gt p - hydr"ox"ybenzoic acid`
D. `m - Hydr"ox"ybenzoic acid gt benzoic acid gt o - hydr"ox"ybenzoic acid gt p - hydr"ox"ybenzoic acid`

Answer

To determine the correct decreasing order of acid strength of hydroxybenzoic acids, we need to analyze the effects of the hydroxyl (-OH) group in different positions relative to the carboxylic acid (-COOH) group on the benzene ring. The three isomers we will consider are ortho-hydroxybenzoic acid (o-HBA), meta-hydroxybenzoic acid (m-HBA), and para-hydroxybenzoic acid (p-HBA).

### Step-by-Step Solution:

1. **Identify the Isomers**:
- Ortho-hydroxybenzoic acid (o-HBA) has the -OH group adjacent to the -COOH group.
- Meta-hydroxybenzoic acid (m-HBA) has the -OH group separated by one carbon from the -COOH group.
- Para-hydroxybenzoic acid (p-HBA) has the -OH group opposite to the -COOH group on the benzene ring.

2. **Analyze the Effects of the -OH Group**:
- The -OH group is an electron-donating group through resonance (it has a +M effect) and can also stabilize the negative charge through hydrogen bonding when the hydrogen ion (H+) is released from the -COOH group.
- In ortho and para positions, the -OH group can stabilize the negative charge on the conjugate base (the anion formed after deprotonation) more effectively than in the meta position.

3. **Consider the Stability of the Conjugate Bases**:
- For o-HBA, the -OH group can engage in intramolecular hydrogen bonding with the carboxylate ion, enhancing stability and thus increasing acidity.
- For p-HBA, the -OH group can stabilize the conjugate base through resonance but lacks the intramolecular hydrogen bonding present in o-HBA.
- For m-HBA, the -OH group does not participate in resonance with the -COOH group, leading to less stabilization of the conjugate base compared to o-HBA and p-HBA.

4. **Rank the Acidity**:
- The strongest acid will be the one that forms the most stable conjugate base.
- The order of acidity based on the stability of the conjugate bases will be:
- **Ortho-hydroxybenzoic acid (o-HBA)** > **Para-hydroxybenzoic acid (p-HBA)** > **Meta-hydroxybenzoic acid (m-HBA)**.

5. **Final Order**:
- Therefore, the correct decreasing order of acid strength of hydroxybenzoic acids is:
- **o-HBA > p-HBA > m-HBA**.