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

Saponification

Organic Chemistry

22. Carboxylic Acid Derivatives: NAS

Saponification

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Problem 58a

Textbook Question

Methyl p-nitrobenzoate has been found to undergo saponification faster than methyl benzoate.
(a) Consider the mechanism of saponification, and explain the reasons for this rate enhancement.

Verified step by step guidance

Understand the saponification mechanism: Saponification is the base-catalyzed hydrolysis of an ester. The reaction involves nucleophilic attack by hydroxide (OH⁻) on the carbonyl carbon of the ester, forming a tetrahedral intermediate, which then collapses to release the alcohol and form the carboxylate ion.

Analyze the structure of methyl p-nitrobenzoate: The ester group is attached to a benzene ring that has a nitro group (-NO₂) in the para position. The nitro group is an electron-withdrawing group (EWG) due to its strong -I (inductive) and -M (mesomeric) effects.

Explain the role of the nitro group: The electron-withdrawing nature of the nitro group stabilizes the partial positive charge on the carbonyl carbon of the ester. This makes the carbonyl carbon more electrophilic and more susceptible to nucleophilic attack by hydroxide ions.

Compare with methyl benzoate: Methyl benzoate lacks the nitro group, so its carbonyl carbon is less electrophilic. As a result, the nucleophilic attack by hydroxide ions occurs more slowly in methyl benzoate compared to methyl p-nitrobenzoate.

Conclude the rate enhancement: The presence of the nitro group in methyl p-nitrobenzoate significantly increases the rate of saponification by making the carbonyl carbon more reactive toward nucleophilic attack, thereby lowering the activation energy of the reaction.

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Key Concepts

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

Saponification Mechanism

Saponification is a nucleophilic substitution reaction where an ester reacts with a base, typically hydroxide ions, to form an alcohol and a carboxylate salt. The mechanism involves the nucleophile attacking the carbonyl carbon of the ester, leading to the formation of a tetrahedral intermediate. Understanding this mechanism is crucial to analyze how substituents on the aromatic ring can influence the reaction rate.

Electronic Effects of Substituents

The presence of electron-withdrawing groups, such as nitro groups, can significantly affect the reactivity of aromatic compounds. In methyl p-nitrobenzoate, the nitro group stabilizes the negative charge that develops during the transition state of the saponification reaction, thereby facilitating the nucleophilic attack. This concept is essential for understanding why methyl p-nitrobenzoate reacts faster than methyl benzoate.

Steric Hindrance

Steric hindrance refers to the repulsion between bulky groups that can impede the approach of nucleophiles to the electrophilic center. In the case of methyl benzoate, the lack of electron-withdrawing groups means that steric factors may play a more significant role in slowing down the reaction. Recognizing the balance between electronic effects and steric hindrance is vital for explaining the observed differences in reaction rates.

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