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 28a
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 28aChapter 22, Problem 28a
Predict the major products of the following base-catalyzed aldol condensations with dehydration.
(a) benzophenone (PhCOPh) + propionaldehyde
Verified step by step guidance1
Identify the enolate-forming compound: In a base-catalyzed aldol condensation, the compound with an alpha-hydrogen will form the enolate ion. In this case, propionaldehyde (CH3CH2CHO) has alpha-hydrogens, while benzophenone (PhCOPh) does not. Therefore, propionaldehyde will form the enolate.
Generate the enolate ion: The base (e.g., OH⁻) will abstract an alpha-hydrogen from propionaldehyde, forming the enolate ion. The enolate ion is a resonance-stabilized species with a negative charge on the alpha-carbon and a delocalized electron pair on the oxygen.
Perform the nucleophilic attack: The enolate ion acts as a nucleophile and attacks the carbonyl carbon of benzophenone (PhCOPh), which is electrophilic. This forms a beta-hydroxy intermediate (a compound with a hydroxyl group on the beta-carbon relative to the original carbonyl group).
Dehydrate the beta-hydroxy intermediate: Under base-catalyzed conditions, the beta-hydroxy intermediate undergoes elimination of water (dehydration) to form an α,β-unsaturated carbonyl compound. This step involves the removal of a hydroxyl group and a beta-hydrogen, resulting in the formation of a double bond between the alpha and beta carbons.
Predict the major product: The final product is an α,β-unsaturated ketone, where the benzophenone moiety (PhCO-) is conjugated with the double bond formed during dehydration. The structure of the product will include the benzophenone group and the extended conjugated system from the propionaldehyde.
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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. This reaction involves the nucleophilic addition of an enolate ion to a carbonyl compound, followed by dehydration to yield an α,β-unsaturated carbonyl compound. Understanding this process is crucial for predicting the products of the reaction.
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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 form a more reactive species. In aldol condensations, a strong base can generate an enolate ion from an aldehyde or ketone, which then acts as a nucleophile. Recognizing the role of the base is essential for understanding how the reaction proceeds and the products formed.
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Dehydration in Organic Reactions
Dehydration in organic chemistry refers to the elimination of water from a compound, often occurring after the formation of an intermediate. In the context of aldol condensation, after the formation of the β-hydroxy carbonyl compound, dehydration leads to the formation of a double bond, resulting in an α,β-unsaturated carbonyl compound. This step is critical for determining the final products of the reaction.
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Related Practice
Textbook Question
Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes.
(c) cyclohexanone
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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
Predict the major products of the following base-catalyzed aldol condensations with dehydration.
(b) 2,2-dimethylpropanal + acetophenone
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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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