Which of the following substituted phenols is the strongest acid?

Options

A.
B.
C.
D.

Answer

To determine which substituted phenol is the strongest acid, we need to analyze the effects of the substituents on the acidity of phenol. The acidity of phenols is influenced by the stability of their conjugate bases. A more stable conjugate base corresponds to a stronger acid.

### Step-by-Step Solution:

1. **Identify the Substituted Phenols**:
We have the following substituted phenols to consider:
- Phenol (C6H5OH)
- Phenol with OCH3 (methoxy group)
- Phenol with NO2 (nitro group)

2. **Understand the Concept of Acidity**:
The acidity of a compound can be understood in terms of its ability to donate a proton (H+). When phenol donates a proton, it forms a phenoxide ion (C6H5O−). The stability of this phenoxide ion is crucial in determining the acidity.

3. **Analyze the Effects of Substituents**:
- **Methoxy Group (OCH3)**: The methoxy group is an electron-donating group (+R effect). It increases the electron density on the aromatic ring, making the phenoxide ion less stable. Thus, phenols with methoxy groups are weaker acids.
- **Nitro Group (NO2)**: The nitro group is an electron-withdrawing group (−R effect). It decreases the electron density on the aromatic ring and stabilizes the phenoxide ion through resonance. This increased stability of the conjugate base indicates that phenols with nitro groups are stronger acids.

4. **Compare the Acidity**:
- Phenol (C6H5OH) has moderate acidity.
- Phenol with OCH3 is less acidic due to the electron-donating effect.
- Phenol with NO2 is the most acidic due to the electron-withdrawing effect, which stabilizes the conjugate base.

5. **Conclusion**:
Based on the analysis, the strongest acid among the given substituted phenols is the one with the nitro group (NO2).

### Final Answer:
The strongest acid among the substituted phenols is the one with the nitro group (NO2).