Textbook Question
Propose mechanisms for the following base-catalyzed condensations, with dehydration.
(a) 2,2-dimethylpropanal with acetaldehyde
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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 28b
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 28bChapter 22, Problem 28b
Predict the major products of the following base-catalyzed aldol condensations with dehydration.
(b) 2,2-dimethylpropanal + acetophenone
Verified step by step guidance1
Step 1: Identify the reactants and their functional groups. In this case, 2,2-dimethylpropanal is an aldehyde, and acetophenone is a ketone. Both contain carbonyl groups (C=O), which are key to the aldol condensation reaction.
Step 2: Determine the enolate formation. Under basic conditions, the alpha-hydrogen (hydrogen attached to the carbon adjacent to the carbonyl group) of acetophenone is abstracted by the base, forming an enolate ion. The enolate ion is nucleophilic and will attack the electrophilic carbonyl carbon of 2,2-dimethylpropanal.
Step 3: Perform the nucleophilic addition step. The enolate ion from acetophenone attacks the carbonyl carbon of 2,2-dimethylpropanal, forming a beta-hydroxy ketone intermediate. This intermediate is the product of the aldol addition step.
Step 4: Dehydration of the beta-hydroxy ketone. Under base-catalyzed conditions, the beta-hydroxy ketone undergoes elimination of water (dehydration) to form an α,β-unsaturated ketone. This step involves the formation of a double bond between the alpha and beta carbons.
Step 5: Predict the major product. The major product will be the α,β-unsaturated ketone formed after dehydration. The structure will include the acetophenone-derived aromatic ring and the double bond introduced during the dehydration step.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Aldol Condensation
Aldol condensation is a reaction between aldehydes or ketones that contain alpha-hydrogens, leading to the formation of β-hydroxy aldehydes or ketones. In the presence of a base, one carbonyl compound is deprotonated to form an enolate ion, which then attacks the carbonyl carbon of another molecule, resulting in a β-hydroxy carbonyl compound. This intermediate can further undergo dehydration to yield an α,β-unsaturated carbonyl compound.
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Base Catalysis
Base catalysis involves the use of a base to facilitate a chemical reaction, often by deprotonating a molecule to generate a reactive intermediate. In aldol condensations, a strong base like sodium hydroxide or potassium hydroxide is used to deprotonate the alpha-hydrogen of the aldehyde or ketone, forming an enolate ion that can attack another carbonyl compound. This step is crucial for the formation of the aldol product.
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Dehydration Reaction
Dehydration is the process of removing a water molecule from a compound, often occurring in organic reactions to form double bonds. In the context of aldol condensation, after the formation of the β-hydroxy carbonyl compound, dehydration occurs, leading to the formation of an α,β-unsaturated carbonyl compound. This step is essential for completing the aldol condensation and is driven by the stability of the resulting conjugated system.
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Related Practice
Textbook Question
Predict the major products of the following base-catalyzed aldol condensations with dehydration.
(a) benzophenone (PhCOPh) + propionaldehyde
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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.
(a)
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Textbook Question
Propose mechanisms for the following base-catalyzed condensations, with dehydration.
(b) benzaldehyde with propionaldehyde
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Textbook Question
When cyclodecane-1,6-dione is treated with sodium carbonate, the product gives a UV spectrum similar to that of 1-acetyl-2-methylcyclopentene. Propose a structure for the product, and give a mechanism for its formation.
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Textbook Question
Show how octane-2,7-dione might cyclize to a cycloheptenone. Explain why ring closure to the cycloheptenone is not favored.
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