Compare the acidic strength of o- and p- nitro phenols.

Answer

To compare the acidic strength of ortho-nitrophenol and para-nitrophenol, we need to analyze the stability of their conjugate bases. The acidic strength of a compound is directly related to the stability of its conjugate base; the more stable the conjugate base, the stronger the acid.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Ortho-nitrophenol (o-Nitrophenol): The nitro group (-NO2) is attached to the benzene ring adjacent to the hydroxyl group (-OH).
- Para-nitrophenol (p-Nitrophenol): The nitro group is attached to the benzene ring opposite the hydroxyl group.

2. **Determine the Conjugate Bases**:
- The conjugate base of ortho-nitrophenol is formed when the hydroxyl group donates a proton (H+), resulting in o-nitrophenoxide ion.
- The conjugate base of para-nitrophenol is formed similarly, resulting in p-nitrophenoxide ion.

3. **Analyze the Effects of the Nitro Group**:
- The nitro group is a strong electron-withdrawing group due to its -I (inductive) and -M (mesomeric) effects. This increases the acidity of the phenols by stabilizing the negative charge on the conjugate base.
- In ortho-nitrophenol, the -NO2 group exerts a stronger -I effect due to its proximity to the -OH group. However, this proximity also leads to steric hindrance and potential hydrogen bonding between the -OH group and the nitro group.

4. **Consider the Stability of Conjugate Bases**:
- In ortho-nitrophenol, the conjugate base may experience destabilization due to intramolecular hydrogen bonding, which makes it less favorable for the proton to dissociate.
- In para-nitrophenol, the conjugate base is more stable because the nitro group can effectively stabilize the negative charge through resonance without the interference of steric hindrance or hydrogen bonding.

5. **Conclusion**:
- While the ortho position has a stronger inductive effect, the destabilization caused by intramolecular hydrogen bonding in ortho-nitrophenol makes it less acidic than para-nitrophenol.
- Therefore, para-nitrophenol is more acidic than ortho-nitrophenol.

### Final Answer:
Para-nitrophenol is more acidic than ortho-nitrophenol due to the greater stability of its conjugate base, which is not affected by intramolecular hydrogen bonding.

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