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

Aldol Condensation

Organic Chemistry

25. Condensation Chemistry

Aldol Condensation

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6:45 minutes

Problem 19c

Textbook Question

Give the expected products for the aldol condensations of (c) pentan-3-one.

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Identify the alpha-hydrogens in pentan-3-one. Alpha-hydrogens are the hydrogens attached to the carbon atoms adjacent to the carbonyl group. In pentan-3-one, the alpha-hydrogens are on carbons 2 and 4.

Determine the enolate ion formation. Under basic conditions, one of the alpha-hydrogens is removed by a base, forming an enolate ion. The enolate ion is stabilized by resonance between the alpha-carbon and the carbonyl oxygen.

Perform the nucleophilic attack. The enolate ion acts as a nucleophile and attacks the carbonyl carbon of another molecule of pentan-3-one, forming a beta-hydroxyketone intermediate.

Dehydrate the beta-hydroxyketone. Under heating or continued basic conditions, the beta-hydroxyketone undergoes elimination of water (E1cB mechanism), forming an α,β-unsaturated ketone as the final product.

Write the final product structure. The product will be a conjugated enone, specifically 4-methylpent-3-en-2-one, resulting from the aldol condensation of pentan-3-one with itself.

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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 carbonyl compounds (aldols) followed by dehydration to yield α,β-unsaturated carbonyl compounds. This reaction is significant in organic synthesis as it allows for the formation of larger carbon skeletons and introduces functional groups.

Enolate Ion Formation

In aldol condensation, the first step involves the formation of an enolate ion from a carbonyl compound, which occurs when a base abstracts an alpha-hydrogen. This enolate ion acts as a nucleophile, attacking the carbonyl carbon of another molecule, leading to the formation of the aldol product. Understanding this step is crucial for predicting the products of the reaction.

Dehydration Reaction

After the aldol product is formed, it can undergo dehydration, where a molecule of water is eliminated, resulting in the formation of an α,β-unsaturated carbonyl compound. This step is essential as it stabilizes the product and enhances its reactivity, making it a key feature of aldol condensations. Recognizing the conditions that favor dehydration is important for predicting the final products.

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Related Practice

Textbook Question

The following compounds can be synthesized by aldol condensations, followed by further reactions. (In each case, work backward from the target molecule to an aldol product, and show what compounds are needed for the condensation.)

(a)

Textbook Question

Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes.

Textbook Question

Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes.

(b) acetophenone

Textbook Question

Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes.

(a) butyraldehyde

Textbook Question

Give the expected products for the aldol condensations of (b) phenylacetaldehyde.

Textbook Question

Give the expected products for the aldol condensations of (a) propanal.

Textbook Question

The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound reacts with an aldehyde or ketone, in the presence of a secondary amine as a basic catalyst, to produce a new C=C. Show the starting materials that made each of these by a Knoevenagel condensation.

(b)

Textbook Question

(c)

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