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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2:06 minutes

Problem 28b(ii)

Textbook Question

b. Explain why the rate of aminolysis of an ester cannot be increased by HO⁻, or RO⁻.

Verified step by step guidance

The aminolysis of an ester involves the reaction of an ester with an amine to form an amide and an alcohol. This reaction proceeds via a nucleophilic acyl substitution mechanism.

In the mechanism, the amine acts as the nucleophile, attacking the carbonyl carbon of the ester. The carbonyl carbon is electrophilic due to the partial positive charge caused by the electronegativity of the oxygen atom in the carbonyl group.

Adding HO⁻ (hydroxide ion) or RO⁻ (alkoxide ion) does not increase the rate of aminolysis because these species are stronger nucleophiles than the amine. If present, they would compete with the amine for the carbonyl carbon, leading to hydrolysis or transesterification instead of aminolysis.

Additionally, the reaction environment for aminolysis is typically neutral or slightly basic to avoid deprotonating the amine. If the solution becomes too basic due to the presence of HO⁻ or RO⁻, the amine may be deprotonated to form an amide ion (RNH⁻), which is a much weaker nucleophile and would slow down the reaction.

Thus, the rate of aminolysis cannot be increased by HO⁻ or RO⁻ because these bases either compete with the amine or alter the reaction conditions in a way that hinders the nucleophilic attack by the amine.

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Key Concepts

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

Aminolysis of Esters

Aminolysis is a nucleophilic substitution reaction where an amine reacts with an ester to form an amide and an alcohol. In this process, the nucleophile (amine) attacks the carbonyl carbon of the ester, leading to the displacement of the leaving group. Understanding the mechanism of this reaction is crucial to analyze how different nucleophiles affect the reaction rate.

Nucleophilicity

Nucleophilicity refers to the strength of a nucleophile in donating an electron pair to form a new bond. In the context of aminolysis, the nucleophile's ability to attack the electrophilic carbonyl carbon of the ester is essential. Hydroxide (HO−) and alkoxide (RO−) ions are strong nucleophiles, but their basicity can lead to competing reactions, which can affect the overall reaction rate.

Basicity vs. Nucleophilicity

Basicity is the tendency of a species to accept protons, while nucleophilicity is its ability to donate electron pairs. In the case of HO− and RO−, their strong basicity can lead to deprotonation of the amine, reducing the effective concentration of the nucleophile needed for aminolysis. This competition between basicity and nucleophilicity explains why increasing the concentration of these bases does not enhance the rate of aminolysis.

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Related Practice

Textbook Question

The reaction of a nitrile with an alcohol in the presence of a strong acid forms an N-substituted amide. This reaction, known as the Ritter reaction, does not work with primary alcohols.

c. How does the Ritter reaction differ from the acid-catalyzed hydrolysis of a nitrile to form an amide?

Textbook Question

a. Which of the following reactions does not give the carbonyl product shown?

b. Which of the reactions that do not occur can be made to occur if an acid catalyst is added to the reaction mixture?

Textbook Question

a. Which of the following reactions does not give the carbonyl product shown?

b. Which of the reactions that do not occur can be made to occur if an acid catalyst is added to the reaction mixture?

10.

Textbook Question

b. Explain why the rate of aminolysis of an ester cannot be increased by H⁺.

Textbook Question

a. Rank the following esters from most reactive to least reactive in the first slow step of a nucleophilic acyl substitution reaction (formation of the tetrahedral intermediate):

Textbook Question

b. Rank the same esters from most reactive to least reactive in the second slow step of a nucleophilic acyl substitution reaction (collapse of the tetrahedral intermediate).

Textbook Question

In each part, rank the compounds in order of increasing rate of nucleophilic attack at C=O by a strong nucleophile like methoxide.

(a)

Textbook Question

Show how you would convert 2-methylcyclopentanol to the following products. Any of these products may be used as the reactant in any subsequent part of this problem.

(g) 2-methylcyclopentyl acetate

(h) 1-bromo-1-methylcyclopentane

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b. Explain why the rate of aminolysis of an ester cannot be increased by HO−, or RO−.

Key Concepts

Aminolysis of Esters

Nucleophilicity

Basicity vs. Nucleophilicity

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b. Explain why the rate of aminolysis of an ester cannot be increased by HO⁻, or RO⁻.