Textbook Question
Propose a mechanism for the following reaction.
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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 58
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 58Chapter 22, Problem 58
The base-catalyzed reaction of an aldehyde (having no α hydrogens) with an anhydride is called the Perkin condensation. Propose a mechanism for the following example of the Perkin condensation. (Sodium acetate serves as the base.)
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Step 1: Identify the reactants and the role of the base. The reaction involves benzaldehyde (an aldehyde with no α-hydrogens) and acetic anhydride. Sodium acetate acts as the base to initiate the reaction by deprotonating acetic anhydride.
Step 2: Deprotonation of acetic anhydride. Sodium acetate abstracts a proton from the methyl group of acetic anhydride, forming an enolate ion. The enolate ion is stabilized by resonance, with the negative charge delocalized between the oxygen atoms and the carbon atom.
Step 3: Nucleophilic attack of the enolate ion on benzaldehyde. The enolate ion acts as a nucleophile and attacks the carbonyl carbon of benzaldehyde, forming a tetrahedral intermediate. This intermediate collapses, regenerating the carbonyl group and forming a β-hydroxy intermediate.
Step 4: Elimination of water to form the α,β-unsaturated product. Under the reaction conditions (heat), the β-hydroxy intermediate undergoes dehydration (loss of water) to form cinnamic acid, which is an α,β-unsaturated carboxylic acid.
Step 5: Formation of acetic acid as a byproduct. During the reaction, acetic acid is formed as a byproduct from the hydrolysis of acetic anhydride and the action of the base. The final products are cinnamic acid and acetic acid.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Perkin Condensation
The Perkin condensation is a reaction between an aldehyde and an anhydride, typically in the presence of a base, to form α,β-unsaturated carboxylic acids. This reaction is significant in organic synthesis as it allows for the formation of carbon-carbon bonds. In this specific case, the aldehyde lacks α-hydrogens, which influences the reaction pathway and product formation.
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Condensation Reactions
Mechanism of Base-Catalyzed Reactions
Base-catalyzed reactions involve the use of a base to facilitate the reaction process, often by deprotonating a substrate to generate a more reactive species. In the context of the Perkin condensation, sodium acetate acts as the base, which helps in the formation of the enolate ion from the aldehyde. This enolate then attacks the anhydride, leading to the formation of the desired product.
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Role of Anhydrides in Organic Reactions
Anhydrides are reactive compounds that can participate in various organic reactions, including acylation and condensation reactions. In the Perkin condensation, the anhydride provides an acyl group that reacts with the enolate formed from the aldehyde. This reaction is crucial for forming the carbon-carbon bond and ultimately leads to the production of α,β-unsaturated carboxylic acids.
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Related Practice
Textbook Question
Show how the following products might be synthesized from suitable Michael donors and acceptors.
(f)
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Textbook Question
Show how the following products might be synthesized from suitable Michael donors and acceptors.
(e)
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Textbook Question
Show how you would use Robinson annulations to synthesize the following compounds. Work backward, remembering that the cyclohexenone is the new ring and that the double bond of the cyclohexenone is formed by the aldol with dehydration. Take apart the double bond, then see what structures the Michael donor and acceptor must have.
(a)
Textbook Question
For each molecule shown below,
1. indicate the most acidic hydrogens.
(a)
(b)
(c)
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Textbook Question
Show how you would use Robinson annulations to synthesize the following compounds. Work backward, remembering that the cyclohexenone is the new ring and that the double bond of the cyclohexenone is formed by the aldol with dehydration. Take apart the double bond, then see what structures the Michael donor and acceptor must have.
(b)
4
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