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 stability of their conjugate bases. The strength of an acid is directly related to the stability of its conjugate base. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Structures
We have four hydroxybenzoic acids to consider:
1. Ortho-hydroxybenzoic acid
2. Meta-hydroxybenzoic acid
3. Para-hydroxybenzoic acid
4. Benzoic acid (for comparison)

### Step 2: Draw the Structures and Conjugate Bases
- **Ortho-hydroxybenzoic acid (salicylic acid)**: The hydroxyl (-OH) group is adjacent to the carboxylic acid (-COOH). The conjugate base will have a negative charge on the oxygen atom.

- **Meta-hydroxybenzoic acid**: The hydroxyl group is positioned one carbon away from the carboxylic acid. The conjugate base will also have a negative charge on the oxygen atom.

- **Para-hydroxybenzoic acid**: The hydroxyl group is directly opposite the carboxylic acid on the benzene ring. The conjugate base will have a negative charge on the oxygen atom.

- **Benzoic acid**: This is the base structure without any hydroxyl group.

### Step 3: Analyze the Effects of Substituents
- **Ortho Position**: The -OH group can participate in intramolecular hydrogen bonding with the carboxylate ion, stabilizing the conjugate base and increasing acidity.

- **Meta Position**: The -OH group exerts a -I effect (inductive effect) but does not stabilize the negative charge effectively through resonance. It provides some stabilization but less than the ortho position.

- **Para Position**: The -OH group provides resonance stabilization to the conjugate base, but the effect is weaker compared to the ortho position due to the distance from the carboxylate group.

### Step 4: Compare Acid Strengths
1. **Ortho-hydroxybenzoic acid**: Strongest acid due to hydrogen bonding stabilizing the conjugate base.
2. **Meta-hydroxybenzoic acid**: Moderate acid strength due to -I effect stabilizing the conjugate base.
3. **Para-hydroxybenzoic acid**: Weaker than meta due to less effective stabilization of the conjugate base.
4. **Benzoic acid**: Weakest as it has no hydroxyl group to stabilize the conjugate base.

### Conclusion: Order of Acid Strength
The correct decreasing order of acid strength is:
**Ortho-hydroxybenzoic acid > Meta-hydroxybenzoic acid > Para-hydroxybenzoic acid > Benzoic acid.**

### Final Answer
The correct order is: **Ortho > Meta > Para > Benzoic acid.**