The strongest acid among the following compounds is:

Options

A. o-nitrophenol
B. p-chlorophenol
C. m-nitrophenol
D. p-nitrophenol

Answer

To determine the strongest acid among the given compounds, we need to analyze the stability of the conjugate base formed after the acid donates a proton (H⁺). The more stable the conjugate base, the stronger the acid will be. Let's break down the analysis step by step:

### Step 1: Identify the Compounds
We have four compounds to analyze:
1. Ortho-Nitrophenol
2. Para-Nitrophenol
3. Meta-Nitrophenol
4. Para-Chlorophenol

### Step 2: Understand Acid Strength
Acids donate protons (H⁺). When an acid donates a proton, it forms a conjugate base (A⁻). The stability of this conjugate base is crucial in determining the strength of the acid. A more stable conjugate base corresponds to a stronger acid.

### Step 3: Analyze Ortho-Nitrophenol
In ortho-nitrophenol, the nitro group is positioned ortho to the hydroxyl (OH) group. When it donates a proton, the resulting conjugate base has a negative charge on the oxygen. The nitro group can stabilize this negative charge through resonance and inductive effects, but the proximity of the nitro group can also lead to hydrogen bonding, which can hinder the release of H⁺.

### Step 4: Analyze Para-Nitrophenol
In para-nitrophenol, the nitro group is positioned para to the hydroxyl group. Similar to ortho-nitrophenol, the nitro group stabilizes the negative charge on the conjugate base through resonance and inductive effects. However, there is no hydrogen bonding in this case, allowing for a more effective release of H⁺ compared to ortho-nitrophenol.

### Step 5: Analyze Meta-Nitrophenol
In meta-nitrophenol, the nitro group is not directly adjacent to the hydroxyl group. The conjugate base formed here is less stabilized by resonance compared to the ortho and para positions. Thus, the acidity will be weaker than both ortho and para-nitrophenol.

### Step 6: Analyze Para-Chlorophenol
In para-chlorophenol, the chlorine atom is an electron-withdrawing group due to its -I effect. However, it does not stabilize the negative charge on the conjugate base as effectively as the nitro group does. Therefore, para-chlorophenol will be less acidic than para-nitrophenol.

### Step 7: Compare Acidity
Now, we compare the stability of the conjugate bases:
- Ortho-Nitrophenol: Stability decreased due to hydrogen bonding.
- Para-Nitrophenol: High stability due to resonance without hydrogen bonding.
- Meta-Nitrophenol: Lower stability due to lack of resonance.
- Para-Chlorophenol: Lower stability due to less effective electron-withdrawing.

### Conclusion
The strongest acid among the given compounds is **Para-Nitrophenol** due to the effective stabilization of its conjugate base without the interference of hydrogen bonding.

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