Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

25. Condensation Chemistry

Crossed Aldol Condensation

Organic Chemistry

25. Condensation Chemistry

Crossed Aldol Condensation

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4:32 minutes

Problem 25a

Textbook Question

Propose mechanisms for the following base-catalyzed condensations, with dehydration.
(a) 2,2-dimethylpropanal with acetaldehyde

Verified step by step guidance

Identify the reaction type: This is a base-catalyzed aldol condensation followed by dehydration. Aldol condensations involve the formation of a β-hydroxy carbonyl compound, which can then undergo dehydration to form an α,β-unsaturated carbonyl compound.

Deprotonation of the α-hydrogen: In the presence of a base (e.g., hydroxide ion, OH⁻), the α-hydrogen of acetaldehyde (CH₃CHO) is removed to form an enolate ion. The enolate ion is stabilized by resonance, with the negative charge delocalized between the oxygen and the α-carbon.

Nucleophilic attack by the enolate: The enolate ion acts as a nucleophile and attacks the carbonyl carbon of 2,2-dimethylpropanal ((CH₃)₃CCHO). This forms a new carbon-carbon bond, resulting in a β-hydroxy intermediate.

Dehydration of the β-hydroxy intermediate: Under basic conditions, the β-hydroxy compound undergoes elimination of water (dehydration). This step involves the removal of a hydroxide ion (OH⁻) and a proton (H⁺) from adjacent carbons, leading to the formation of a double bond between the α and β carbons.

Final product: The result is an α,β-unsaturated carbonyl compound, where the double bond is conjugated with the carbonyl group, providing stability to the product.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Base-Catalyzed Condensation

Base-catalyzed condensation reactions involve the formation of a new bond between two carbonyl compounds, facilitated by a base. The base deprotonates an alpha hydrogen, generating a nucleophile that attacks the carbonyl carbon of another molecule, leading to the formation of a β-hydroxy carbonyl compound. This intermediate can then undergo dehydration to yield an α,β-unsaturated carbonyl compound.

Mechanism of Aldol Reaction

The aldol reaction is a key mechanism in organic chemistry where two aldehydes or ketones react in the presence of a base to form a β-hydroxy aldehyde or ketone. The mechanism involves the formation of an enolate ion from one carbonyl compound, which then attacks the carbonyl carbon of another compound. This reaction is crucial for understanding the formation of larger carbon skeletons and the subsequent dehydration step that leads to unsaturated products.

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 reactions, after the formation of the β-hydroxy carbonyl compound, dehydration results in the formation of a double bond, yielding an α,β-unsaturated carbonyl compound. This step is essential for driving the reaction towards more stable products and is often facilitated by heat or the presence of an acid or base.

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