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

22. Carboxylic Acid Derivatives: NAS

Nucleophilic Acyl Substitution

Organic Chemistry

22. Carboxylic Acid Derivatives: NAS

Nucleophilic Acyl Substitution

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3:19 minutes

Problem 6c

Textbook Question

Propose a second mechanism for the reaction of benzoic acid with acetyl chloride to give acetic benzoic anhydride. This time, let the other oxygen of benzoic acid serve as the nucleophile to attack the carbonyl group of acetyl chloride. Because proton transfers are fast between these oxygen atoms, it is difficult to differentiate between these two mechanisms experimentally.

Verified step by step guidance

Step 1: Begin by identifying the reactants and their roles. Benzoic acid (C₆H₅COOH) has two oxygen atoms: one in the hydroxyl group (-OH) and one in the carbonyl group (C=O). Acetyl chloride (CH₃COCl) contains a highly electrophilic carbonyl carbon due to the electron-withdrawing effect of the chlorine atom.

Step 2: In this proposed mechanism, the carbonyl oxygen of benzoic acid will act as the nucleophile. The lone pair of electrons on the carbonyl oxygen attacks the electrophilic carbonyl carbon of acetyl chloride, forming a tetrahedral intermediate. Represent this step as:

C_{6} H_{5} C = O + CH 3_{-} COCl \to tetrahedral intermediate

Step 3: The tetrahedral intermediate is unstable and undergoes a rearrangement. The chloride ion (Cl⁻) is expelled as a leaving group, and a proton transfer occurs between the hydroxyl group and the newly formed oxygen atom. This step stabilizes the intermediate and forms the acetic benzoic anhydride product.

Step 4: Highlight the role of proton transfer. Proton transfers between the hydroxyl oxygen and the carbonyl oxygen of benzoic acid are fast and reversible. This makes it challenging to experimentally distinguish between this mechanism and the one where the hydroxyl oxygen acts as the nucleophile.

Step 5: Conclude by noting that both mechanisms lead to the same product, acetic benzoic anhydride. The difference lies in which oxygen atom of benzoic acid initiates the nucleophilic attack on acetyl chloride.

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Key Concepts

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

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution is a fundamental reaction in organic chemistry where a nucleophile attacks the electrophilic carbon of a carbonyl group, leading to the substitution of a leaving group. In the context of benzoic acid and acetyl chloride, the nucleophile (the oxygen atom of benzoic acid) attacks the carbonyl carbon of acetyl chloride, forming a tetrahedral intermediate that eventually leads to the formation of an anhydride.

Proton Transfer Mechanism

Proton transfer mechanisms involve the movement of protons (H⁺ ions) between atoms, which can significantly influence reaction pathways. In the proposed reaction, rapid proton transfers between the oxygen atoms of benzoic acid can complicate the identification of distinct mechanisms, as these transfers can occur before the reaction products are formed, making it challenging to track the specific roles of each oxygen atom.

Carbonyl Reactivity

The reactivity of carbonyl compounds is a key concept in organic chemistry, as the carbonyl carbon is electrophilic and susceptible to nucleophilic attack. In the reaction between benzoic acid and acetyl chloride, the carbonyl group of acetyl chloride is particularly reactive due to the presence of the electronegative chlorine atom, which enhances the electrophilicity of the carbonyl carbon, facilitating the nucleophilic attack by the benzoic acid.

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