Which is the following is the strongest acid ?

Options

A. 3,5-dinitrophenol
B. 2,4-dinitrophenol
C. phenol
D. 2,4,6-trinitrophenol

Answer

To determine which of the following compounds is the strongest acid, we need to analyze the structures and the effects of substituents on the acidity of phenolic compounds. Here’s a step-by-step solution:

### Step 1: Identify the Compounds
We have the following compounds to consider:
1. 3,4-Dinitrophenol
2. 2,4-Dinitrophenol
3. 2,4,6-Trinitrophenol
4. Phenol

### Step 2: Draw the Structures
We can draw the structures of these compounds:
- **3,4-Dinitrophenol**: A benzene ring with two nitro groups at the 3 and 4 positions and a hydroxyl (OH) group.
- **2,4-Dinitrophenol**: A benzene ring with two nitro groups at the 2 and 4 positions and a hydroxyl (OH) group.
- **2,4,6-Trinitrophenol**: A benzene ring with three nitro groups at the 2, 4, and 6 positions and a hydroxyl (OH) group.
- **Phenol**: A simple benzene ring with one hydroxyl (OH) group.

### Step 3: Understand the Effect of Nitro Groups
Nitro groups (NO2) are strong electron-withdrawing groups due to their -I (inductive) effect. The presence of these groups increases the acidity of phenolic compounds by stabilizing the negative charge on the phenoxide ion (the conjugate base formed after deprotonation).

### Step 4: Analyze the Position of Nitro Groups
The position of the nitro groups relative to the hydroxyl group is crucial:
- **Ortho and Para positions**: Nitro groups at these positions are more effective in increasing acidity due to resonance and inductive effects.
- **Meta position**: Nitro groups at this position are less effective.

### Step 5: Compare the Acidity
- **Phenol** has no nitro groups and is the least acidic.
- **3,4-Dinitrophenol** has two nitro groups, which enhance its acidity.
- **2,4-Dinitrophenol** also has two nitro groups, but their positions may have a different effect.
- **2,4,6-Trinitrophenol** has three nitro groups, which maximizes the electron-withdrawing effect, making it the strongest acid among the options.

### Conclusion
Based on the analysis, **2,4,6-Trinitrophenol** is the strongest acid because it has the highest number of nitro groups in the most effective positions for enhancing acidity.

### Final Answer
**The strongest acid among the given options is 2,4,6-Trinitrophenol.**

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