p-Nitrophenol (pK_a = 7.2) is ten times more acidic than m-nitrophenol (pK_a = 8.4) Explain.

Verified step by step guidance

1. Step 1: Understand the relationship between pKa and acidity. A lower pKa value indicates a stronger acid. Since p-nitrophenol has a pKa of 7.2 and m-nitrophenol has a pKa of 8.4, p-nitrophenol is more acidic.
2. Step 2: Analyze the structures of p-nitrophenol and m-nitrophenol. Both molecules contain a phenol group (-OH attached to a benzene ring) and a nitro group (-NO2). The position of the nitro group relative to the hydroxyl group differs: para (p) for p-nitrophenol and meta (m) for m-nitrophenol.
3. Step 3: Consider the electron-withdrawing effect of the nitro group. The nitro group is highly electronegative and withdraws electron density from the benzene ring through both inductive and resonance effects. In p-nitrophenol, the nitro group is positioned para to the hydroxyl group, allowing resonance stabilization of the phenoxide ion formed after deprotonation.
4. Step 4: Examine resonance stabilization in p-nitrophenol. When p-nitrophenol loses a proton, the negative charge on the oxygen can delocalize through the benzene ring and interact with the nitro group via resonance. This delocalization stabilizes the phenoxide ion, making p-nitrophenol more acidic.
5. Step 5: Compare with m-nitrophenol. In m-nitrophenol, the nitro group is not in a position to participate in resonance stabilization of the phenoxide ion. The electron-withdrawing effect is limited to inductive effects, which are weaker than the combined inductive and resonance effects in p-nitrophenol. This explains why m-nitrophenol is less acidic than p-nitrophenol.

Key Concepts

Acidity and pKa

Acidity in organic chemistry is often measured using the pKa value, which indicates the strength of an acid in solution. A lower pKa value corresponds to a stronger acid, meaning it more readily donates protons (H+). In this context, p-nitrophenol has a pKa of 7.2, making it a stronger acid than m-nitrophenol, which has a pKa of 8.4.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, leading to a more stable overall structure. In p-nitrophenol, the nitro group is positioned para to the hydroxyl group, allowing for effective resonance that stabilizes the negative charge on the phenoxide ion formed after deprotonation. This stabilization is less effective in m-nitrophenol due to its meta positioning.

Inductive Effect

The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents on a molecule, influencing its reactivity and acidity. The nitro group in p-nitrophenol exerts a strong electron-withdrawing inductive effect, enhancing the acidity by stabilizing the negative charge on the conjugate base. In contrast, the inductive effect in m-nitrophenol is less pronounced, contributing to its lower acidity.