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

Acid-Catalyzed Alpha-Halogentation

Organic Chemistry

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Acid-Catalyzed Alpha-Halogentation

Previous problemNext problem

5:17 minutes

Problem 10

Textbook Question

A ketone undergoes acid-catalyzed bromination, acid-catalyzed chlorination, racemization, and acid-catalyzed deuterium exchange at the ⍺-carbon. All of these reactions have similar rate constants.What does this tell you about the mechanisms of these reactions?

Verified step by step guidance

The first step is to recognize that all the reactions mentioned (acid-catalyzed bromination, chlorination, racemization, and deuterium exchange) occur at the α-carbon of the ketone. This suggests that the α-carbon is the reactive site in all these processes.

Next, consider the role of the acid catalyst. The acid protonates the carbonyl oxygen of the ketone, increasing the electrophilicity of the carbonyl carbon and making the α-hydrogen more acidic. This facilitates the formation of an enol intermediate or an enolate ion under acidic conditions.

Understand that the enol (or enolate) intermediate is common to all these reactions. The similar rate constants for all the reactions indicate that the rate-determining step is the same for each reaction, which is the formation of the enol intermediate from the ketone.

For bromination and chlorination, the enol reacts with the halogen (Br₂ or Cl₂) to form the halogenated product. For racemization, the enol tautomerizes back to the ketone, potentially leading to a loss of stereochemical integrity. For deuterium exchange, the enol reacts with D₂O, replacing the α-hydrogen with deuterium.

Finally, conclude that the similar rate constants for all these reactions strongly suggest that the formation of the enol intermediate is the rate-determining step, and the subsequent steps (halogenation, tautomerization, or exchange) are faster and do not significantly affect the overall reaction rate.

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.

Mechanism of Electrophilic Aromatic Substitution

In electrophilic aromatic substitution reactions, an electrophile replaces a hydrogen atom on an aromatic ring. The rate of these reactions is influenced by the stability of the carbocation intermediate formed during the process. Understanding this mechanism is crucial for analyzing how ketones react under acidic conditions, as the presence of electron-withdrawing groups can affect the reactivity and orientation of the electrophile.

Acid-Catalyzed Reactions

Acid-catalyzed reactions involve the use of an acid to increase the rate of a chemical reaction. In the context of ketones, the acid can protonate the carbonyl oxygen, enhancing the electrophilicity of the carbonyl carbon. This makes the carbon more susceptible to nucleophilic attack, which is essential for understanding the bromination and chlorination processes mentioned in the question.

Racemization and Stereochemistry

Racemization refers to the process where a chiral molecule converts into a mixture of its enantiomers, resulting in a loss of optical activity. In the context of the reactions described, the similar rate constants suggest that the mechanisms leading to racemization and deuterium exchange involve similar transition states or intermediates. This indicates that the stereochemical outcomes of these reactions are closely related, providing insight into the underlying reaction pathways.

Watch next

Master Acid Catalyzed with a bite sized video explanation from Johnny

Start learning