Textbook Question
Many of the condensations we have studied are reversible. The reverse reactions are often given the prefix retro-, the Latin word meaning “backward.” Propose mechanisms to account for the following reactions.
(c)
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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 80b
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 80bChapter 22, Problem 80b
Many of the condensations we have studied are reversible. The reverse reactions are often given the prefix retro-, the Latin word meaning “backward.” Propose mechanisms to account for the following reactions.
(b) 
Verified step by step guidance1
Step 1: Recognize that the reaction involves a retro-aldol reaction followed by further condensation. Retro-aldol reactions occur under acidic or basic conditions and involve the cleavage of a β-hydroxy ketone or aldehyde into two smaller carbonyl compounds.
Step 2: Analyze the starting material, which is a β-hydroxy ketone. Under acidic conditions (H⁺), the hydroxyl group (-OH) is protonated, making it a better leaving group.
Step 3: After protonation, the molecule undergoes cleavage at the β-carbon, breaking the C-C bond between the α-carbon and β-carbon. This results in the formation of two smaller fragments: a ketone and an aldehyde.
Step 4: The aldehyde fragment formed in the retro-aldol reaction undergoes further condensation. In this case, the aldehyde reacts with the ketone fragment via an aldol condensation mechanism, forming a new C-C bond and producing an α,β-unsaturated ketone.
Step 5: The final product is the α,β-unsaturated ketone shown in the image. This product is stabilized by conjugation between the double bond and the carbonyl group, which lowers its energy and makes it favorable.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Reversible Reactions
Reversible reactions are chemical processes that can proceed in both forward and backward directions. In organic chemistry, this means that the products can react to form the original reactants under certain conditions. Understanding the equilibrium between reactants and products is crucial for predicting the direction of the reaction and the conditions that favor either the forward or reverse process.
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EAS Reactions Overview
Mechanism of Reactions
A reaction mechanism is a step-by-step description of how reactants transform into products at the molecular level. It includes the sequence of bond-breaking and bond-forming events, intermediates, and transition states. Proposing a mechanism requires knowledge of the types of reactions involved, such as nucleophilic substitutions or eliminations, and the role of catalysts or solvents in influencing the reaction pathway.
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Retro Reactions
Retro reactions, often denoted with the prefix 'retro-', refer to the reverse of a condensation reaction, where larger molecules break down into smaller ones. This concept is particularly relevant in organic synthesis, where understanding how to revert a product back to its reactants can provide insights into reaction conditions and the stability of intermediates. Recognizing the factors that drive retro reactions is essential for proposing accurate mechanisms.
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Related Practice
Textbook Question
Write equations showing the expected products of the following enamine alkylation and acylation reactions. Then give the final products expected after hydrolysis of the iminium salts.
(d) piperidine enamine of cyclopentanone + methyl vinyl ketone
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Textbook Question
Many of the condensations we have studied are reversible. The reverse reactions are often given the prefix retro-, the Latin word meaning “backward.” Propose mechanisms to account for the following reactions.
(a)
1
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Textbook Question
Propose a mechanism for the following reaction.
2
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Textbook Question
Show how you would accomplish the following multistep conversions. You may use any additional reagents you need.
(a)
3
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Textbook Question
(A true story.) Chemistry lab students added an excess of ethylmagnesium bromide to methyl furoate, expecting the Grignard reagent to add twice and form the tertiary alcohol. After water workup, they found that the product was a mixture of two compounds. One was the expected product having two ethyl groups, but the unexpected product had added three ethyl groups. Propose a mechanism to explain the formation of the unexpected product.
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