Which one of the following is most acidic ?

Options

A.
B.
C.
D.

Answer

To determine which compound is the most acidic among the given options, we need to analyze the acidity of phenol and its substituted derivatives based on the stability of the resulting phenoxide ion after deprotonation.

### Step-by-Step Solution:

1. **Understanding Acidity**:
- Acidity is defined by the ability of a compound to donate a proton (H⁺). When phenol (C₆H₅OH) loses a proton, it forms a phenoxide ion (C₆H₅O⁻).

2. **Formation of Phenoxide Ion**:
- The phenoxide ion is stabilized by the delocalization of the negative charge. The more stable the phenoxide ion, the stronger the acid.

3. **Role of Substituents**:
- The stability of the phenoxide ion can be affected by substituents on the benzene ring. Electron-withdrawing groups (EWGs) stabilize the negative charge on the phenoxide ion by pulling electron density away from it.

4. **Identifying Electron-Withdrawing Groups**:
- In the given options, we will analyze the substituents:
- **Nitro group (−NO₂)**: This is a strong electron-withdrawing group due to its -I (inductive effect) and -M (mesomeric effect).
- **Chloro group (−Cl)**: This is also an electron-withdrawing group but is less effective than the nitro group.
- **Methoxy group (−OCH₃)**: This group is an electron-donating group due to its +M effect, which would destabilize the phenoxide ion.

5. **Comparing Acidity**:
- The presence of a nitro group will significantly stabilize the phenoxide ion due to its strong electron-withdrawing nature.
- The chloro group provides some stabilization but is not as effective as the nitro group.
- The methoxy group, being electron-donating, would decrease the acidity of the phenol.

6. **Conclusion**:
- Among the substituents analyzed, the nitro group provides the greatest stabilization to the phenoxide ion, making it the most acidic. Therefore, the compound with the nitro group will be the most acidic.

### Final Answer:
The most acidic compound is the one with the **nitro group (−NO₂)**.

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