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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2:05 minutes

Problem 31a

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)

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Identify the structure of the allylic radical. The image shows two resonance structures of an allylic radical, where the unpaired electron is delocalized over the allylic position.

Understand the concept of resonance. Resonance in organic chemistry refers to the delocalization of electrons across adjacent atoms, which stabilizes the molecule. In allylic radicals, the unpaired electron can be shared between the two carbon atoms adjacent to the double bond.

Determine the most stable position for the new bond formation. In the case of allylic radicals, the new bond is most likely to form at the carbon atom where the radical is most stable due to resonance stabilization.

Consider the electronic effects. The formation of the C–Br bond will occur at the carbon atom that can best stabilize the positive charge that results from the radical's electron pairing with the bromine atom.

Select the carbon atom for bond formation. Based on resonance structures, the new C–Br bond is most likely to form at the carbon atom that is part of the more stable resonance structure, which is typically the less substituted carbon in the allylic position.

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Key Concepts

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

Allylic Radical Stability

Allylic radicals are stabilized by resonance, where the unpaired electron is delocalized over adjacent pi bonds. This delocalization increases the stability of the radical, making allylic positions favorable sites for reactions. Understanding this stability is crucial for predicting where new bonds will form in radical reactions.

Resonance Structures

Resonance structures depict the delocalization of electrons within a molecule, showing different possible configurations. In the case of allylic radicals, resonance allows the radical to be distributed over multiple carbon atoms, influencing the site of bond formation. Recognizing resonance structures helps in identifying the most reactive sites in a molecule.

Radical Propagation Steps

Radical propagation involves the transfer of an unpaired electron to form new bonds, typically in a chain reaction. In the second propagation step, the radical reacts with another molecule, such as Br2, to form a new C–Br bond. Understanding the mechanism of propagation steps is essential for predicting the outcome of radical reactions.

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Related Practice

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

Predict the products of the following allylic halogenation reactions.

(d)

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.

Textbook Question

Draw the products of the following reactions, including all stereoisomers:

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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)

Key Concepts

Allylic Radical Stability

Resonance Structures

Radical Propagation Steps

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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)