Textbook Question
Explain why we speak of acidic hydrolysis of an ester as acid-catalyzed, but of basic hydrolysis as base-promoted.
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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 16a
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.
Verified step by step guidance1
Step 1: Analyze the structure of (R)-2-butyl acetate labeled with O-18. The molecule consists of an ester functional group, where the oxygen atom labeled with O-18 is part of the ester bond. Hydrolysis under basic conditions involves breaking the ester bond to form an alcohol and a carboxylate ion.
Step 2: Write the mechanism for the hydrolysis under basic conditions. The hydroxide ion (OH⁻) attacks the carbonyl carbon of the ester, forming a tetrahedral intermediate. Represent this step using curved arrows to show the nucleophilic attack and bond formation.
Step 3: The tetrahedral intermediate collapses, expelling the alkoxide group (butan-2-ol) and forming the carboxylate ion. The O-18 label remains attached to the carboxylate ion because the bond between the labeled oxygen and the carbonyl carbon is not broken during the reaction.
Step 4: Predict the stereochemistry of the butan-2-ol product. Under basic conditions, the alkoxide group can rotate freely, leading to racemization. Thus, the butan-2-ol product will be a racemic mixture of (R)- and (S)-enantiomers.
Step 5: Summarize the results. The O-18 label will be found in the carboxylate ion product, and the butan-2-ol product will be racemized due to the loss of stereochemical integrity during the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydrolysis Mechanism
Hydrolysis is a chemical reaction involving the breaking of a bond in a molecule using water. In the context of esters, such as (R)-2-butyl acetate, hydrolysis typically occurs under acidic or basic conditions, leading to the formation of an alcohol and a carboxylic acid. The mechanism involves nucleophilic attack by hydroxide ions on the carbonyl carbon, followed by the breakdown of the ester bond.
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Isotope Labeling
Isotope labeling involves substituting a specific atom in a molecule with a heavier isotope, such as O-18 in this case. This technique is useful in tracking the fate of atoms during chemical reactions. In the hydrolysis of (R)-2-butyl acetate, the O-18 label will be retained in the product that forms from the ester bond, allowing for the identification of which product contains the labeled oxygen.
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Stereochemistry and Racemization
Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the hydrolysis of (R)-2-butyl acetate, the reaction can lead to racemization, where the product mixture contains both (R) and (S) enantiomers. The presence of a strong nucleophile and the nature of the reaction can influence whether the product remains optically pure or becomes a racemic mixture.
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Related Practice
Textbook Question
Propose a mechanism for the following ring-opening transesterification. Use the mechanism in Problem 21-13 as a model.
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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.
(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
Propose a mechanism for the acid-catalyzed hydrolysis of phenylalanine ethyl ester.
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Textbook Question
1. Propose a mechanism for the acid-catalyzed reaction of salicylic acid with acetic anhydride.
2. Explain why a single drop of sulfuric acid dramatically increases the reaction rate.
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