Introduction

Para hydroxy benzoic acid (p-Hydroxybenzoic acid) and para nitro phenol (p-Nitrophenol) are both organic compounds with different functional groups. The acidity of these compounds can be compared by analyzing their respective acid dissociation constants (pKa values). In this discussion, we will explain why para hydroxy benzoic acid is more acidic than para nitro phenol.

Explanation

1. Structure and Resonance Effects

- Para hydroxy benzoic acid contains a hydroxyl group (-OH) attached to the benzene ring, while para nitro phenol has a nitro group (-NO2) in place of the hydroxyl group.
- The hydroxyl group in p-Hydroxybenzoic acid can easily donate a proton (H+) due to the electronegativity difference between oxygen and hydrogen.
- The nitro group in p-Nitrophenol, on the other hand, is a strong electron-withdrawing group due to the presence of three electronegative oxygen atoms. This group decreases the electron density on the phenolic oxygen, making it less likely to donate a proton.

2. Inductive Effects

- The presence of the nitro group in p-Nitrophenol also induces a strong inductive effect, withdrawing electrons from the phenolic oxygen. This further stabilizes the conjugate base of p-Nitrophenol.
- In contrast, p-Hydroxybenzoic acid does not have such a strong electron-withdrawing group, resulting in a higher electron density on the phenolic oxygen. This makes it easier for the compound to donate a proton.

3. Resonance Stabilization

- The phenolic oxygen in both compounds can undergo resonance stabilization. However, the presence of the nitro group in p-Nitrophenol reduces the extent of resonance due to electron-withdrawing effects.
- In p-Hydroxybenzoic acid, the hydroxyl group can donate an electron pair to the benzene ring, resulting in resonance stabilization. This contributes to the stability of the conjugate base, making it more acidic.

4. Acid Dissociation Constant (pKa) Comparison

- The acid dissociation constant (pKa) is a measure of the acidity of a compound. Lower pKa values indicate stronger acidity.
- The pKa value of p-Hydroxybenzoic acid is around 4.5, while the pKa value of p-Nitrophenol is approximately 7.2.
- The lower pKa value of p-Hydroxybenzoic acid indicates its greater tendency to donate a proton and suggests that it is more acidic than p-Nitrophenol.

Conclusion

In conclusion, para hydroxy benzoic acid is more acidic than para nitro phenol due to several factors. The presence of a hydroxyl group, absence of strong electron-withdrawing groups, and greater resonance stabilization contribute to the higher acidity of p-Hydroxybenzoic acid. Conversely, the presence of a nitro group and its strong electron-withdrawing effects reduce the acidity of p-Nitrophenol.

Becz. it has carboxylic group which is more acidic than phenol