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

19. Reactions of Aromatics: EAS and Beyond

Electron Withdrawing Groups

Organic Chemistry

19. Reactions of Aromatics: EAS and Beyond

Electron Withdrawing Groups

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Problem 50

Textbook Question

In Chapter 23, we learned about the electrophilic aromatic nitration reaction. When phenol is subjected to these conditions with a large excess of nitric acid, a molecule called picric acid is produced. Predict the product of this reaction and explain why the pK_a value of this compound is 0.38.

Verified step by step guidance

Step 1: Recognize that phenol (C₆H₅OH) undergoes electrophilic aromatic nitration when treated with concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄). The hydroxyl (-OH) group on phenol is an activating group, making the aromatic ring highly reactive towards electrophilic substitution reactions.

Step 2: Understand the mechanism of nitration. The reaction involves the generation of the nitronium ion (NO₂⁺) as the electrophile, which is formed by the interaction of HNO₃ and H₂SO₄. The nitronium ion attacks the aromatic ring, leading to substitution at positions that are ortho and para to the hydroxyl group due to its electron-donating nature.

Step 3: Note that under the reaction conditions (large excess of nitric acid and elevated temperature), multiple nitration occurs. Phenol reacts to form 2,4,6-trinitrophenol, commonly known as picric acid. The hydroxyl group directs the nitro groups to the ortho and para positions relative to itself.

Step 4: Explain the pKa value of picric acid (0.38). The presence of three electron-withdrawing nitro groups (-NO₂) significantly increases the acidity of the hydroxyl group. These nitro groups stabilize the negative charge on the phenoxide ion formed after deprotonation, making picric acid a very strong acid compared to phenol.

Step 5: Summarize the reaction: Phenol reacts with concentrated HNO₃ and H₂SO₄ at 100°C to produce picric acid (2,4,6-trinitrophenol). The low pKa value is due to the strong electron-withdrawing effects of the nitro groups, which enhance the acidity of the compound.

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Key Concepts

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

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of nitration, the electrophile is the nitronium ion (NO₂⁺), which attacks the electron-rich aromatic system. Understanding EAS is crucial for predicting the products of reactions involving aromatic compounds, such as the formation of picric acid from phenol.

Acidity and pKₐ

The pKₐ value is a measure of the acidity of a compound, indicating how easily it donates a proton (H⁺) in solution. A lower pKₐ value signifies a stronger acid. Picric acid, with a pKₐ of 0.38, is highly acidic due to the presence of multiple electron-withdrawing nitro groups, which stabilize the negative charge on the conjugate base formed after deprotonation, making it more likely to release a proton.

Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons across multiple structures, which can enhance the stability of a molecule. In picric acid, the negative charge on the conjugate base can be delocalized over the nitro groups, leading to increased stability. This stabilization contributes to the compound's strong acidity, as the more stable the conjugate base, the more favorable the deprotonation process.

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