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.
(b)
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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 80a
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 80aChapter 22, Problem 80a
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) 
Verified step by step guidance1
Step 1: Recognize that the reaction is a retro-aldol reaction, which involves the cleavage of a β-hydroxy ketone or aldehyde into two smaller carbonyl compounds under acidic or basic conditions.
Step 2: Protonate the hydroxyl group (-OH) of the β-hydroxy ketone using the acidic conditions (H+). This makes the hydroxyl group a better leaving group.
Step 3: Form a carbocation by eliminating water (H2O) from the protonated hydroxyl group. This generates a resonance-stabilized intermediate.
Step 4: Break the C-C bond between the α-carbon and β-carbon, resulting in the formation of two carbonyl compounds: one ketone and one aldehyde.
Step 5: Ensure the products are correctly identified as CH3C(O)(CH2)5CHO (aldehyde) and CH3C(O) (ketone), consistent with the reverse of the aldol condensation reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Retro-Aldol Reaction
The retro-aldol reaction is the reverse of the aldol condensation, where a β-hydroxy carbonyl compound breaks down into two carbonyl compounds. This reaction typically occurs under acidic or basic conditions and is characterized by the cleavage of the carbon-carbon bond formed during the aldol reaction. Understanding this mechanism is crucial for predicting the products of the reaction shown in the question.
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Crossed Aldol
Aldol Condensation
Aldol condensation is a reaction between aldehydes or ketones that contain at least one α-hydrogen, leading to the formation of β-hydroxy carbonyl compounds. This process involves the nucleophilic addition of an enolate ion to a carbonyl carbon, followed by dehydration to yield an α,β-unsaturated carbonyl compound. Recognizing the aldol condensation mechanism helps in understanding how the retro-aldol reaction can occur.
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Acid-Base Catalysis
Acid-base catalysis involves the use of acids or bases to accelerate chemical reactions. In the context of retro-aldol reactions, an acid can protonate the hydroxyl group, making it a better leaving group and facilitating the cleavage of the carbon-carbon bond. This concept is essential for understanding how the reaction conditions influence the equilibrium between the aldol and retro-aldol products.
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Related Practice
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)
1134
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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
855
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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.
(c) piperidine enamine of cyclopentanone + methyl iodide
983
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Textbook Question
Show how you would accomplish the following multistep conversions. You may use any additional reagents you need.
(a)
1348
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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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