Textbook Question
Propose a mechanism for the base-promoted hydrolysis of γ-butyrolactone:
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Ch. 21 - Carboxylic Acid Derivatives
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 21, Problem 17a
Explain why we speak of acidic hydrolysis of an ester as acid-catalyzed, but of basic hydrolysis as base-promoted.
Verified step by step guidance1
Understand the difference between a catalyst and a promoter: A catalyst speeds up a reaction without being consumed, while a promoter is a substance that enhances the reaction but may be consumed in the process.
For acidic hydrolysis of an ester, recognize that the acid acts as a catalyst. The acid donates a proton to the ester, making the carbonyl carbon more electrophilic and susceptible to nucleophilic attack by water. The acid is regenerated at the end of the reaction, so it is not consumed.
For basic hydrolysis of an ester (also called saponification), note that the base (e.g., hydroxide ion, OH⁻) is consumed in the reaction. The hydroxide ion directly attacks the carbonyl carbon of the ester, breaking the bond and forming a carboxylate ion and an alcohol.
Explain why the term 'base-promoted' is used: Since the base is consumed in the reaction and is not regenerated, it is not acting as a catalyst. Instead, it promotes the reaction by participating as a reactant.
Summarize the distinction: Acidic hydrolysis is acid-catalyzed because the acid is regenerated, while basic hydrolysis is base-promoted because the base is consumed during the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid-Catalyzed Hydrolysis
Acid-catalyzed hydrolysis of esters involves the addition of an acid, typically a strong acid like hydrochloric acid, which donates protons (H+) to the reaction. This protonation of the ester enhances its electrophilicity, making it more susceptible to nucleophilic attack by water. The term 'catalyzed' indicates that the acid is not consumed in the reaction; it merely facilitates the process by lowering the activation energy.
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Acid Catalyzed
Base-Promoted Hydrolysis
In contrast, base-promoted hydrolysis of esters involves the use of a base, such as sodium hydroxide, which provides hydroxide ions (OH-) that directly attack the ester. This process is termed 'promoted' because the base is consumed in the reaction, forming a carboxylate ion and alcohol. The term emphasizes the role of the base in driving the reaction forward rather than merely facilitating it.
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Mechanistic Differences
The mechanistic differences between acid-catalyzed and base-promoted hydrolysis are crucial for understanding their terminology. Acid-catalyzed reactions typically proceed through a protonation step that activates the ester, while base-promoted reactions involve nucleophilic attack by hydroxide ions. Recognizing these mechanisms helps clarify why the terms 'catalyzed' and 'promoted' are used differently, reflecting the distinct roles of acids and bases in these hydrolysis reactions.
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Related Practice
Textbook Question
Suppose we have some optically pure (R)-2-butyl acetate that has been 'labeled' with the heavy 18O isotope at one oxygen atom as shown.
(c) Explain how you would prove experimentally where the 18O label appears in the products. (18O is not radioactive.)
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Textbook Question
Suppose we have some optically pure (R)-2-butyl acetate that has been "labeled" with the heavy 18O isotope at one oxygen atom as shown.
(b) Repeat part (a) for the acid-catalyzed hydrolysis of this compound.
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Textbook Question
Soap manufacturers always use base to hydrolyze fats, and never acid. Suggest two reasons that basic hydrolysis is preferred.
Textbook Question
Draw the important resonance contributors for both resonance-stabilized cations (in brackets) in the mechanism for acid-catalyzed hydrolysis of an amide.
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Textbook Question
Suppose we have some optically pure (R)-2-butyl acetate that has been 'labeled' with the heavy 18O isotope at one oxygen atom as shown.
(a) Draw a mechanism for the hydrolysis of this compound under basic conditions. Predict which of the products will contain the 18O label. Also predict whether the butan-2-ol product will be pure (R), pure (S), or racemized.
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