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

11. Radical Reactions

Allylic Bromination

Organic Chemistry

11. Radical Reactions

Allylic Bromination

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2:48 minutes

Problem 32d

Textbook Question

Predict the products of the following allylic halogenation reactions.
(d)

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Identify the allylic position in the given molecule. The allylic position is the carbon atom adjacent to the double bond. In this structure, the allylic position is the carbon atom next to the double bond on the left side.

Understand the role of NBS (N-bromosuccinimide) in the reaction. NBS is used for allylic bromination, which selectively replaces a hydrogen atom at the allylic position with a bromine atom.

Consider the reaction conditions. The presence of light indicates a radical mechanism, which is typical for allylic bromination with NBS.

Generate the allylic radical. The reaction begins with the abstraction of an allylic hydrogen atom by a bromine radical, forming an allylic radical.

Predict the product. The allylic radical will react with a bromine molecule to form the allylic bromide. The bromine atom will attach to the allylic position, resulting in the formation of the allylic brominated product.

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Key Concepts

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

Allylic Halogenation

Allylic halogenation is a reaction where a halogen is introduced at the allylic position of an alkene. This position is adjacent to the double bond, allowing for resonance stabilization of the intermediate radical. The reaction typically involves a radical mechanism, initiated by light or heat, and is useful for functionalizing alkenes.

N-Bromosuccinimide (NBS)

N-Bromosuccinimide (NBS) is a reagent commonly used for allylic bromination. It provides a controlled source of bromine radicals, which selectively react at the allylic position. NBS is preferred for its ability to minimize over-bromination and its effectiveness in radical reactions, especially under light or heat conditions.

Radical Mechanism

The radical mechanism involves the formation of radical intermediates, which are highly reactive species with unpaired electrons. In allylic halogenation, radicals are generated by the homolytic cleavage of bonds, often initiated by light. These radicals propagate the reaction by abstracting hydrogen atoms and forming new bonds, leading to the substitution at the allylic position.

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Related Practice

Textbook Question

Propose mechanisms to account for the observed products in the following reactions.

(b)

Textbook Question

Show the products you expect when each compound reacts with NBS with light shining on the reaction.

(a)

(b)

Textbook Question

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

a. Propose a mechanism for this reaction.

Textbook Question

When methylenecyclohexane is treated with a low concentration of bromine under irradiation by a sunlamp, two substitution products are formed.

a. Propose structures for these two products. (b) Propose a mechanism to account for their formation.

Textbook Question

In the following allylic radicals, identify the carbon where the new C–Br bond is most likely to form in the second propagation step.

(a)

Textbook Question

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

d. Explain why cyclohexene reacts with bromine much faster than cyclohexane, which must be heated to react.

Textbook Question

In the presence of a small amount of bromine, the following light-promoted reaction has been observed.

a. Write a mechanism for this reaction. Your mechanism should explain how both products are formed. (Hint: Notice which H atom has been lost in both products.)

Textbook Question

The fact that allylic halogenation results in formation of the most stable alkene suggests that it is under thermodynamic control. Thus, the second propagation step must be reversible. Suggest an arrow-pushing mechanism by which the less stable allylic halide might equilibrate to the more stable allylic halide.

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Predict the products of the following allylic halogenation reactions.(d)

Key Concepts

Allylic Halogenation

N-Bromosuccinimide (NBS)

Radical Mechanism

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Predict the products of the following allylic halogenation reactions.
(d)