Explain why the pK_a of p-nitrophenol is 7.14, whereas the pK_a of m-nitrophenol is 8.39.

Verified step by step guidance

1. Understand the concept of pKa: The pKa value is a measure of the acidity of a compound. A lower pKa indicates a stronger acid, meaning the compound more readily donates a proton (H⁺). The acidity is influenced by the stability of the conjugate base formed after deprotonation.
2. Analyze the structure of p-nitrophenol: In p-nitrophenol, the nitro group (-NO₂) is located at the para position relative to the hydroxyl group (-OH). The nitro group is an electron-withdrawing group due to its strong -I (inductive) and -M (mesomeric) effects. At the para position, the -M effect allows resonance stabilization of the conjugate base by delocalizing the negative charge on the oxygen atom of the phenoxide ion.
3. Analyze the structure of m-nitrophenol: In m-nitrophenol, the nitro group is located at the meta position relative to the hydroxyl group. At this position, the nitro group can only exert an inductive (-I) effect, as the -M effect does not contribute to resonance stabilization of the conjugate base. This makes the conjugate base of m-nitrophenol less stabilized compared to p-nitrophenol.
4. Compare the stabilization of the conjugate bases: The conjugate base of p-nitrophenol is more stabilized due to the resonance effect of the nitro group at the para position. In contrast, the conjugate base of m-nitrophenol is less stabilized because the nitro group at the meta position cannot participate in resonance. Greater stabilization of the conjugate base leads to a stronger acid (lower pKa).
5. Conclude the reasoning: Since the conjugate base of p-nitrophenol is more stabilized than that of m-nitrophenol, p-nitrophenol is a stronger acid, resulting in a lower pKa (7.14) compared to m-nitrophenol (8.39).

Key Concepts

Acidity and pKa

The pKa value is a quantitative measure of the acidity of a compound, indicating the strength of an acid in solution. A lower pKa value corresponds to a stronger acid, meaning it dissociates more readily to release protons (H+). Understanding pKa is essential for comparing the acidity of different compounds, such as p-nitrophenol and m-nitrophenol.

Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons across multiple structures, which can enhance the stability of a conjugate base formed after deprotonation. In the case of p-nitrophenol, the nitro group is positioned para to the hydroxyl group, allowing for effective resonance stabilization of the phenoxide ion, thus increasing its acidity compared to m-nitrophenol, where the resonance is less effective.

Substituent Effects on Acidity

The position and nature of substituents on a benzene ring can significantly influence the acidity of phenolic compounds. Electron-withdrawing groups, like nitro groups, increase acidity by stabilizing the negative charge on the conjugate base. The difference in pKa values between p-nitrophenol and m-nitrophenol can be attributed to the varying effects of the nitro group’s position on the overall electron distribution and stability of the resulting phenoxide ions.