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

Nucleophilic Aromatic Substitution

Organic Chemistry

19. Reactions of Aromatics: EAS and Beyond

Nucleophilic Aromatic Substitution

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4:30 minutes

Problem 11

Textbook Question

How do the mechanisms of the following reactions differ?

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Step 1: Analyze the first reaction (i). The substrate is an aniline (benzene ring with an -NH2 group), and the reagent is Cl2. This reaction involves electrophilic aromatic substitution, where the chlorine acts as an electrophile and substitutes a hydrogen atom on the benzene ring. The -NH2 group is an activating group and directs the substitution to the ortho and para positions due to its electron-donating nature.

Step 2: Examine the second reaction (ii). The substrate is a bromopyridine (pyridine ring with a bromine atom), and the reagent is NaOCH3. This reaction involves nucleophilic aromatic substitution, where the bromine atom is replaced by the methoxy group (-OCH3). Pyridine is electron-deficient due to the nitrogen atom, making it susceptible to nucleophilic attack. The reaction proceeds via a Meisenheimer complex intermediate.

Step 3: Compare the mechanisms. Reaction (i) proceeds via an electrophilic aromatic substitution mechanism, which involves the generation of a carbocation intermediate stabilized by resonance. The -NH2 group enhances the reactivity of the benzene ring toward electrophiles. Reaction (ii), on the other hand, proceeds via nucleophilic aromatic substitution, which involves the formation of a negatively charged intermediate (Meisenheimer complex) before the bromine is displaced.

Step 4: Highlight the role of substituents. In reaction (i), the -NH2 group is an electron-donating group that activates the benzene ring and directs substitution to the ortho and para positions. In reaction (ii), the nitrogen atom in pyridine withdraws electron density, making the ring more susceptible to nucleophilic attack, and the bromine atom serves as a good leaving group.

Step 5: Summarize the key difference. The primary distinction between the two reactions is the type of substitution mechanism: electrophilic aromatic substitution in reaction (i) versus nucleophilic aromatic substitution in reaction (ii). The nature of the substituents and the reagents determines the mechanism and the outcome of each reaction.

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Key Concepts

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

Nucleophilic Aromatic Substitution (NAS)

Nucleophilic aromatic substitution is a reaction mechanism where a nucleophile replaces a leaving group on an aromatic ring. This process typically occurs when the aromatic compound has electron-withdrawing groups that stabilize the negative charge developed during the reaction. Understanding this mechanism is crucial for analyzing how different nucleophiles and leaving groups affect the reaction pathway and product formation.

Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution is a mechanism where an electrophile replaces a hydrogen atom on an aromatic ring. In the presence of a strong electrophile, such as Cl2, the aromatic system undergoes a temporary disruption of its aromaticity, allowing for substitution. This concept is essential for understanding how reactions with halogens differ from those involving nucleophiles, as seen in the provided reactions.

Leaving Groups

Leaving groups are atoms or groups that can depart from a molecule during a chemical reaction, facilitating the substitution process. The ability of a leaving group to stabilize the negative charge after departure is critical in determining the reaction's feasibility and rate. In the context of the reactions shown, the nature of the leaving group (Cl in the first reaction and Br in the second) influences the mechanism and outcome of the nucleophilic aromatic substitution.

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