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

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Tautomers of Dicarbonyl Compounds

Organic Chemistry

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Tautomers of Dicarbonyl Compounds

Previous problemNext problem

4:06 minutes

Problem 6

Textbook Question

Explain why 92% of 2,4-pentanedione exists as the enol tautomer in hexane but only 15% of this compound exists as the enol tautomer in water.

Verified step by step guidance

Understand the concept of keto-enol tautomerism: Keto-enol tautomerism is a chemical equilibrium between a ketone (or aldehyde) and its corresponding enol form. The equilibrium depends on the stability of each tautomer, which is influenced by the solvent and other factors.

Analyze the structure of 2,4-pentanedione: This compound has two carbonyl groups (C=O) separated by a single methylene group (-CH2-). The enol form involves the migration of a hydrogen atom and the formation of a double bond between the alpha-carbon and one of the carbonyl carbons, along with the formation of an -OH group.

Consider the role of the solvent hexane: Hexane is a nonpolar solvent. In nonpolar environments, the enol form of 2,4-pentanedione is stabilized by intramolecular hydrogen bonding between the hydroxyl (-OH) group and the adjacent carbonyl oxygen. This intramolecular hydrogen bonding significantly increases the stability of the enol tautomer in hexane.

Consider the role of the solvent water: Water is a polar solvent and can form hydrogen bonds with the keto form of 2,4-pentanedione. The keto form is more polar than the enol form, so it is better stabilized by the polar environment of water. Additionally, the intramolecular hydrogen bonding that stabilizes the enol form in hexane is less significant in water due to competition with water molecules for hydrogen bonding.

Conclude the reasoning: The high percentage of the enol tautomer in hexane (92%) is due to the stabilization provided by intramolecular hydrogen bonding in the nonpolar solvent. In contrast, the lower percentage of the enol tautomer in water (15%) is due to the polar solvent stabilizing the keto form more effectively through hydrogen bonding and solvation effects.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Video duration:

Play a video:

Was this helpful?

Key Concepts

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

Tautomerism

Tautomerism is a chemical phenomenon where two or more structural isomers, known as tautomers, readily interconvert. In the case of 2,4-pentanedione, the keto form and the enol form are tautomers. The stability of these forms can vary significantly depending on the solvent and environmental conditions, influencing the equilibrium between them.

Solvent Effects on Tautomerism

The solvent plays a crucial role in determining the stability of tautomers. In nonpolar solvents like hexane, the enol form of 2,4-pentanedione is favored due to reduced hydrogen bonding and steric interactions, allowing for a higher percentage of the enol tautomer. Conversely, in polar solvents like water, the keto form is stabilized through hydrogen bonding, leading to a lower percentage of the enol form.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to an electronegative atom interacts with another electronegative atom. In water, the ability of the solvent to form extensive hydrogen bonds stabilizes the keto form of 2,4-pentanedione, thus reducing the proportion of the enol tautomer compared to nonpolar solvents where such interactions are minimal.

Watch next

Master Tautomers of Dicarbonyls with a bite sized video explanation from Johnny

Start learning