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

Tautomerization

Organic Chemistry

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Tautomerization

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6:17 minutes

Problem 2a

Textbook Question

Show each step in the mechanism of the acid-catalyzed interconversion of (R)- and (S)-3-methylpentan-2-one.

Verified step by step guidance

Step 1: Protonation of the carbonyl oxygen - In the presence of an acid catalyst, the lone pair of electrons on the oxygen atom of the carbonyl group in 3-methylpentan-2-one interacts with a proton (H⁺). This results in the formation of a protonated carbonyl intermediate, making the carbonyl carbon more electrophilic.

Step 2: Formation of the enol intermediate - A base (such as water or the conjugate base of the acid) abstracts a proton from the alpha-carbon (the carbon adjacent to the carbonyl group). This deprotonation leads to the formation of a double bond between the alpha-carbon and the carbonyl carbon, while the oxygen atom retains a lone pair of electrons, forming an enol intermediate.

Step 3: Reprotonation of the enol - The enol intermediate undergoes protonation at the oxygen atom, resulting in the formation of a protonated enol. This step is reversible and depends on the acidic environment.

Step 4: Tautomerization to the ketone - The protonated enol undergoes tautomerization, where the double bond between the alpha-carbon and the carbonyl carbon is broken, and a proton is transferred to the alpha-carbon. This regenerates the carbonyl group, forming the ketone. During this step, the stereochemistry at the alpha-carbon can invert, leading to the interconversion of the (R)- and (S)-enantiomers.

Step 5: Deprotonation to regenerate the catalyst - The final step involves the deprotonation of the protonated carbonyl group by a base, regenerating the acid catalyst and yielding the neutral ketone product. This completes the acid-catalyzed interconversion of (R)- and (S)-3-methylpentan-2-one.

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Key Concepts

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

Acid-Catalyzed Mechanism

An acid-catalyzed mechanism involves the use of an acid to facilitate a chemical reaction. In this context, the acid donates a proton (H+) to the carbonyl oxygen of the ketone, increasing its electrophilicity. This step is crucial for the subsequent nucleophilic attack by water or alcohol, leading to the formation of a tetrahedral intermediate, which can then rearrange to yield the desired enantiomer.

Stereochemistry and Enantiomers

Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, such as (R)- and (S)-3-methylpentan-2-one. Understanding the stereochemical implications of the reaction is essential for analyzing how the acid-catalyzed mechanism leads to the interconversion of these two enantiomers.

Tetrahedral Intermediate

A tetrahedral intermediate is a transient species formed during nucleophilic addition to a carbonyl compound. In the case of the acid-catalyzed interconversion, the addition of water to the protonated carbonyl creates this intermediate, which can undergo further reactions. Recognizing the formation and stability of this intermediate is key to understanding the mechanism and the pathway leading to the final products.

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