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

EAS:Friedel-Crafts Acylation Mechanism

Organic Chemistry

19. Reactions of Aromatics: EAS and Beyond

EAS:Friedel-Crafts Acylation Mechanism

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5:03 minutes

Problem 4

Textbook Question

Show the mechanism for the generation of the acylium ion if an acid anhydride is used instead of an acyl chloride for the source of the acylium ion.

Verified step by step guidance

Step 1: Recognize that the acid anhydride is the source of the acylium ion. Acid anhydrides are compounds with two acyl groups bonded to an oxygen atom. In this case, the acid anhydride reacts with a Lewis acid catalyst, AlCl₃, to generate the acylium ion.

Step 2: The Lewis acid, AlCl₃, interacts with one of the carbonyl oxygen atoms in the acid anhydride. This interaction increases the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack.

Step 3: A cleavage occurs in the acid anhydride. The bond between the carbonyl carbon and the oxygen atom breaks, resulting in the formation of the acylium ion (R-C≡O⁺) and a carboxylate ion (R-COO⁻). The acylium ion is stabilized by resonance, where the positive charge is delocalized between the carbon and oxygen atoms.

Step 4: The carboxylate ion formed in the previous step can coordinate with the AlCl₃ catalyst, forming a complex. This complex stabilizes the carboxylate ion and prevents it from interfering with the reaction.

Step 5: The acylium ion is now available to participate in electrophilic aromatic substitution reactions, such as Friedel-Crafts acylation, where it reacts with the benzene ring to form an aromatic ketone.

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Key Concepts

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

Acylium Ion Formation

The acylium ion is a resonance-stabilized cation formed from acyl compounds, such as acyl chlorides or acid anhydrides. In the presence of a Lewis acid like AlCl3, the carbonyl carbon of the acyl compound becomes electrophilic, allowing for the generation of the acylium ion. This ion is crucial in electrophilic aromatic substitution reactions, particularly in Friedel-Crafts acylation.

Friedel-Crafts Acylation

Friedel-Crafts acylation is a reaction that introduces an acyl group into an aromatic ring using an acylium ion. The process involves the electrophilic attack of the acylium ion on the aromatic system, followed by deprotonation to restore aromaticity. This reaction is significant for synthesizing ketones from aromatic compounds and is facilitated by Lewis acids that activate the acylium ion.

Acid Anhydrides vs. Acyl Chlorides

Acid anhydrides and acyl chlorides are both sources of acylium ions, but they differ in reactivity and byproducts. Acid anhydrides are generally less reactive than acyl chlorides, leading to milder reaction conditions. When using acid anhydrides, the reaction produces a carboxylic acid as a byproduct, while acyl chlorides yield hydrochloric acid. Understanding these differences is essential for predicting reaction outcomes in acylation processes.

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