Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

Ranking Acidity

Organic Chemistry

3. Acids and Bases

Ranking Acidity

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6:44 minutes

Problem 23

Textbook Question

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

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.

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Related Practice

Textbook Question

Which acid in each of the following pairs is stronger?

Textbook Question

Purine is a heterocyclic compound with four nitrogen atoms.

a. Which nitrogen is most apt to be protonated?

b. Which nitrogen is least apt to be protonated?

Textbook Question

Which acid in each of the following pairs is stronger?

Textbook Question

For each of the following substituents, indicate whether it withdraws electrons inductively, donates electrons by hyperconjugation, withdraws electrons by resonance, or donates electrons by resonance.

d. NHCH₃

e. OCH₃

f. ⁺N(CH₃)₃

Textbook Question

a. Br

b. CH₂CH₃

Textbook Question

Which loses a proton more readily: a methyl group bonded to cyclohexane or a methyl group bonded to benzene?

Textbook Question

A nitro group (–NO₂) effectively stabilizes a negative charge on an adjacent carbon atom through resonance:

Two of the following nitrophenols are much more acidic than phenol itself. The third compound is only slightly more acidic than phenol. Use resonance structures of the appropriate phenoxide ions to show why two of these anions should be unusually stable.

Textbook Question

In each pair, choose the most basic compound. Justify your answer.

(d)

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