Textbook Question
Propose a mechanism for the acid-catalyzed reaction of cyclohexanone with ethylene glycol to give cyclohexanone ethylene acetal.
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Ch. 18 - Ketones and Aldehydes
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 18, Problem 27
In the mechanism for acetal hydrolysis shown, the ring oxygen atom was protonated first, the ring was cleaved, and then the methoxy group was lost. The mechanism could also be written to show the methoxy oxygen protonating and cleaving first, followed by ring cleavage. Draw this alternative mechanism.
Verified step by step guidance1
Identify the starting structure of the acetal and recognize the functional groups involved. Acetals typically have two alkoxy groups (-OR) attached to the same carbon atom, and in this case, one of the alkoxy groups is part of a ring structure.
Protonate the methoxy oxygen atom first. This step involves the addition of a proton (H⁺) to the methoxy oxygen, making it a better leaving group. Represent this step using curved arrows to show the movement of electrons from the oxygen lone pair to the proton.
Facilitate the cleavage of the methoxy group. After protonation, the methoxy group becomes a good leaving group and departs, forming a carbocation intermediate. Use curved arrows to show the bond breaking between the carbon and the methoxy group.
Protonate the ring oxygen atom. Once the methoxy group has left, the ring oxygen atom can be protonated by another molecule of acid (H⁺), making it more electrophilic and susceptible to cleavage.
Cleave the ring structure. After protonation, the bond between the ring oxygen and the adjacent carbon is broken, leading to the formation of a second carbocation intermediate and the release of the ring oxygen as a neutral molecule. Use curved arrows to show the bond breaking and electron movement.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acetal Hydrolysis
Acetal hydrolysis is a chemical reaction where an acetal is converted back into its corresponding carbonyl compound and alcohol in the presence of water, typically under acidic conditions. This process involves the protonation of the acetal oxygen, leading to the cleavage of the carbon-oxygen bond and the formation of a hemiacetal intermediate before yielding the final products.
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Acetal Protecting Group
Protonation
Protonation is the addition of a proton (H+) to a molecule, which often enhances its reactivity. In the context of acetal hydrolysis, protonation of the oxygen atom in the acetal increases the electrophilicity of the carbon atom, facilitating the cleavage of the ring and the subsequent loss of the methoxy group.
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Reaction Mechanism
A reaction mechanism is a step-by-step description of the pathway by which reactants are converted into products. Understanding the mechanism of acetal hydrolysis is crucial, as it allows chemists to visualize the sequence of protonation, bond cleavage, and rearrangement, which can vary depending on the order of events, such as whether the methoxy group or the ring oxygen is protonated first.
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Related Practice
Textbook Question
Show what alcohols and carbonyl compounds give the following derivatives.
(a)
(b)
(c)
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Textbook Question
Propose a mechanism for the acid-catalyzed hydrolysis of cyclohexanone dimethyl acetal.
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Textbook Question
Propose a mechanism for the acid-catalyzed hydrolysis of cyclohexanone ethylene acetal.
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Textbook Question
Propose a mechanism for the acid-catalyzed hydrolysis of the acetal given in Problem 18-26(f).
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Textbook Question
Propose a mechanism for the acid-catalyzed reaction of benzaldehyde with methanol to give benzaldehyde dimethyl acetal.
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