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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4:13 minutes

Problem 11

Textbook Question

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

Verified step by step guidance

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.

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.

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.

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.

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.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.

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