Textbook Question
Esters with only one α hydrogen generally give poor yields in the Claisen condensation. Propose a mechanism for the Claisen condensation of ethyl isobutyrate, and explain why a poor yield is obtained.
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Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds
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
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Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 33b
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 33bChapter 22, Problem 33b
The following compound results from base-catalyzed aldol cyclization of a 2-substituted cyclohexanone.
(b) Propose a mechanism for the cyclization.
Verified step by step guidance1
Step 1: Identify the starting material and the product. The starting material is a 2-substituted cyclohexanone, and the product is a bicyclic compound formed via aldol cyclization. The key structural feature is the formation of a new carbon-carbon bond between the alpha-carbon of the ketone and another carbon within the molecule.
Step 2: Deprotonation of the alpha-carbon. In the presence of a base, the alpha-hydrogen of the cyclohexanone is removed, forming an enolate ion. The enolate is stabilized by resonance, with the negative charge delocalized between the alpha-carbon and the oxygen atom of the carbonyl group. The enolate is the nucleophile in this reaction.
Step 3: Intramolecular nucleophilic attack. The enolate ion attacks the electrophilic carbonyl carbon within the same molecule, forming a new carbon-carbon bond. This step results in the formation of a cyclic intermediate, which is a beta-hydroxy ketone.
Step 4: Dehydration of the beta-hydroxy ketone. Under basic conditions, the beta-hydroxy ketone undergoes elimination of water to form an alpha, beta-unsaturated ketone. This step involves the removal of a proton from the beta-carbon and the hydroxyl group, leading to the formation of a double bond.
Step 5: Final structure confirmation. The product is a bicyclic compound with a conjugated double bond and a ketone functional group. The mechanism demonstrates how the base-catalyzed aldol cyclization leads to the formation of this structure.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Aldol Reaction
The aldol reaction is a fundamental organic reaction where an enolate ion reacts with a carbonyl compound to form a β-hydroxy aldehyde or ketone. This reaction is crucial in forming carbon-carbon bonds and is often used in the synthesis of larger, more complex molecules. In the context of cyclization, the aldol reaction can lead to the formation of cyclic structures when the β-hydroxy compound undergoes dehydration.
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Base-Catalyzed Mechanism
In a base-catalyzed mechanism, a strong base deprotonates a carbonyl compound to form an enolate ion, which is a nucleophile. This enolate then attacks another carbonyl carbon, leading to the formation of a β-hydroxy carbonyl compound. The mechanism typically involves several steps, including enolate formation, nucleophilic attack, and subsequent dehydration to form an alkene, which is essential for understanding the cyclization process.
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Cyclization
Cyclization refers to the process of forming a cyclic compound from a linear precursor. In the context of aldol reactions, cyclization occurs when the β-hydroxy carbonyl compound undergoes intramolecular reactions, leading to the formation of a ring structure. This process is significant in organic synthesis as it can create complex cyclic molecules that are often found in natural products and pharmaceuticals.
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Related Practice
Textbook Question
Ethoxide is used as the base in the condensation of ethyl acetate to avoid some unwanted side reactions. Show what side reactions would occur if the following bases were used.
(a) sodium methoxide
(b) sodium hydroxide
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Textbook Question
The following compound results from base-catalyzed aldol cyclization of a 2-substituted cyclohexanone.
(a) Show the diketone that would cyclize to give this product.
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Textbook Question
Show how each compound can be dissected into reagents joined by an aldol condensation, then decide whether the necessary aldol condensation is feasible.
(e)
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Textbook Question
Predict the products of self-condensation of the following esters.
(a) methyl propanoate + NaOCH3
(b) ethyl phenylacetate + NaOCH2CH3
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Textbook Question
Show how each compound can be dissected into reagents joined by an aldol condensation, then decide whether the necessary aldol condensation is feasible.
(c)
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