Textbook Question
Predict the products of self-condensation of the following esters.
(d)
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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 35
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 35Chapter 22, Problem 35
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.
Verified step by step guidance1
Step 1: Begin by understanding the Claisen condensation mechanism. The Claisen condensation involves the reaction of two ester molecules in the presence of a strong base, typically an alkoxide, to form a β-keto ester. The reaction requires at least one α-hydrogen on the ester to proceed effectively.
Step 2: Analyze the structure of ethyl isobutyrate. Ethyl isobutyrate has only one α-hydrogen attached to the carbon adjacent to the ester group. This α-hydrogen is necessary for enolate ion formation, which is the key intermediate in the Claisen condensation.
Step 3: Describe the enolate formation step. The base abstracts the α-hydrogen from ethyl isobutyrate, forming an enolate ion. The enolate ion is resonance-stabilized, with the negative charge delocalized between the oxygen atom of the ester group and the α-carbon. Represent this step using the formula:
Step 4: Explain the nucleophilic attack step. The enolate ion attacks another molecule of ethyl isobutyrate at the carbonyl carbon, forming a tetrahedral intermediate. This intermediate then collapses, expelling the alkoxide group and forming the β-keto ester product.
Step 5: Discuss why the yield is poor. Ethyl isobutyrate has only one α-hydrogen, which limits the formation of enolate ions. Additionally, steric hindrance from the isobutyrate group reduces the efficiency of the nucleophilic attack. These factors contribute to the poor yield in the Claisen condensation of ethyl isobutyrate.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Claisen Condensation
Claisen condensation is a reaction between two esters or an ester and a carbonyl compound in the presence of a strong base, leading to the formation of a β-keto ester or a β-diketone. The mechanism involves the deprotonation of an α-hydrogen by the base, generating an enolate ion that attacks the carbonyl carbon of another ester, resulting in the formation of a new carbon-carbon bond.
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Claisen Condensation
α Hydrogen
α hydrogens are the hydrogen atoms attached to the carbon atom adjacent to a carbonyl group. Their presence is crucial in reactions like the Claisen condensation, as they can be deprotonated to form enolate ions. Esters with fewer α hydrogens, such as those with bulky or branched groups, are less likely to form enolates, leading to lower reactivity and poor yields in the condensation reaction.
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Steric Hindrance
Steric hindrance refers to the repulsion between bulky groups in a molecule that can impede reactions. In the case of ethyl isobutyrate, the presence of a bulky isobutyl group can hinder the approach of the base to the α hydrogen, making it more difficult to form the enolate. This steric effect contributes to the poor yield observed in the Claisen condensation of esters with limited α hydrogens.
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Related Practice
Textbook Question
The following compound results from base-catalyzed aldol cyclization of a 2-substituted cyclohexanone.
(b) Propose a mechanism for the cyclization.
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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
989
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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
Predict the products of self-condensation of the following esters.
(c)
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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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