Textbook Question
Which acid in each of the following pairs is stronger?
e.
f.
3. Acids and Bases
Ranking Acidity
6:44 minutesProblem 23
Textbook Question
Explain why the pKa of p-nitrophenol is 7.14, whereas the pKa of m-nitrophenol is 8.39.
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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.
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.
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.
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).
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).
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6mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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.
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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.
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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.
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