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

Haloform Reaction

Organic Chemistry

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Haloform Reaction

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4:14 minutes

Problem 58

Textbook Question

In the presence of excess base and excess halogen, a methyl ketone is converted to a carboxylate ion. The reaction is known as the haloform reaction because one of the products is haloform (chloroform, bromoform, or iodoform). Before spectroscopy became a routine analytical tool, the haloform reaction served as a test for methyl ketones: the formation of iodoform, a bright yellow compound, signaled that a methyl ketone was present. Why do only methyl ketones form a haloform?

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The haloform reaction occurs when a methyl ketone reacts with a halogen (Cl₂, Br₂, or I₂) in the presence of a base. The reaction involves multiple steps, including halogenation and cleavage of the carbon-carbon bond adjacent to the carbonyl group.

The key structural feature required for the haloform reaction is the presence of a methyl group directly attached to the carbonyl carbon. This methyl group is essential because it allows for the formation of a trihalogenated intermediate during the reaction.

In the first step, the base deprotonates the alpha-hydrogen of the methyl ketone, forming an enolate ion. This enolate ion is highly nucleophilic and reacts with the halogen to form a halogenated intermediate. This process repeats until the methyl group is fully halogenated, forming a trihalomethyl ketone.

Once the trihalomethyl ketone is formed, the base attacks the carbonyl carbon, leading to cleavage of the carbon-carbon bond. This produces a carboxylate ion and a trihalomethane (haloform, such as chloroform, bromoform, or iodoform).

Only methyl ketones undergo this reaction because the methyl group provides the necessary hydrogens for complete halogenation. Other ketones, such as those with longer alkyl chains, do not have the required structure to form the trihalogenated intermediate and thus cannot produce a haloform.

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Key Concepts

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

Haloform Reaction

The haloform reaction is a chemical reaction where a methyl ketone reacts with a halogen in the presence of a base to produce a haloform (such as chloroform, bromoform, or iodoform) and a carboxylate ion. This reaction is significant in organic chemistry as it serves as a qualitative test for the presence of methyl ketones, which can be identified by the formation of the characteristic yellow iodoform.

Methyl Ketones

Methyl ketones are a specific class of ketones characterized by having a methyl group (–CH3) adjacent to the carbonyl group (C=O). This structural feature is crucial for the haloform reaction, as it allows for the necessary deprotonation and subsequent halogenation steps that lead to the formation of haloform products. Other ketones do not have this arrangement, which is why they do not undergo the haloform reaction.

Base Catalysis

In the haloform reaction, a strong base (such as sodium hydroxide) is essential for deprotonating the methyl ketone, generating an enolate ion. This enolate ion is highly reactive and facilitates the halogenation process, ultimately leading to the formation of the haloform. The presence of excess base ensures that the reaction proceeds efficiently, highlighting the importance of base catalysis in organic reactions.

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