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

10. Addition Reactions

Halogenation

Organic Chemistry

10. Addition Reactions

Halogenation

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Problem 54

Textbook Question

In Chapter 19, we discuss the reaction of enols with bromine. This reaction produces α -bromoketones in good yields. Suggest a mechanism for this reaction and justify its deviation from the dibromide product you might have expected.

Verified step by step guidance

Step 1: Recognize that the reaction involves enols reacting with bromine (Br₂) to form α-bromoketones. The enol tautomer is the reactive intermediate in this process, and the reaction proceeds via electrophilic halogenation.

Step 2: Understand the mechanism begins with the enol donating electrons from its double bond to bromine (Br₂), forming a bromonium ion intermediate. This step is driven by the nucleophilicity of the enol's double bond.

Step 3: The bromonium ion is then attacked by water or another nucleophile, leading to the formation of a hydroxyl group and a bromine atom on the α-carbon. This intermediate is unstable and undergoes further transformation.

Step 4: The hydroxyl group on the α-carbon is eliminated as water, resulting in the formation of a carbonyl group (ketone). This step explains why the final product is an α-bromoketone rather than a dibromide.

Step 5: The deviation from the dibromide product occurs because the reaction mechanism favors the formation of a stable carbonyl group over the addition of a second bromine atom. The reaction stops at the α-bromoketone stage due to the stability of the ketone product.

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Key Concepts

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

Enol Tautomerization

Enols are the keto-enol tautomer of carbonyl compounds, where the enol form contains a hydroxyl group bonded to a carbon-carbon double bond. Understanding enol tautomerization is crucial because the reaction with bromine involves the enol form, which is more reactive than the keto form. This process highlights the dynamic equilibrium between the two forms and the conditions that favor one over the other.

Electrophilic Addition

Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, resulting in the formation of a new bond. In the case of enols reacting with bromine, the double bond of the enol acts as a nucleophile, attacking the electrophilic bromine molecule. This step is essential for understanding how α-bromoketones are formed instead of the expected dibromide product.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In the reaction of enols with bromine, the regioselectivity is influenced by the stability of the resulting intermediates and products. This concept helps explain why the reaction favors the formation of α-bromoketones rather than a dibrominated product, as the more stable product is formed preferentially.

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Textbook Question

Predict the product(s) of each of the following reactions, making sure to indicate the relative stereochemical outcome. Indicate any racemic mixtures by drawing both enantiomers.

(b)

Textbook Question

Predict the product(s) of each of the following reactions, making sure to indicate the relative stereochemical outcome. Indicate any racemic mixtures by drawing both enantiomers.

(f)

Textbook Question

In which of the two steps in the alkene halogenation mechanism does a redox reaction occur?

Textbook Question

Bromination of a highly electron-rich alkene such as 2-methoxybut-2-ene has been shown to produce approximately equal mixtures of the trans- and cis-dibromide. Suggest an explanation for this observation.

Textbook Question

Identify A through O:

Textbook Question

Predict the product of the following addition reactions to dienes.

(c)

Textbook Question

Predict the product(s) that would result when the alkenes are allowed to react under the following conditions: (i) Br₂ (ii) Cl₂.

(e)

Textbook Question

Provide an arrow-pushing mechanism that rationalizes the stereospecific formation of each dihalide in Assessment 9.6.

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