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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4:51 minutes

Problem 71

Textbook Question

The halogenation of an alkane when there is an alkene present in the molecule does not proceed with the regioselectivity you might expect. Using principles similar to those developed in this chapter, rationalize the formation of A as the only product. We study this reaction further in Chapter 8.

Verified step by step guidance

Identify the functional groups present in the molecule: The molecule contains both an alkane and an alkene. Alkanes undergo free radical halogenation, while alkenes typically undergo electrophilic addition reactions. Recognizing these functional groups is key to understanding the reaction pathway.

Analyze the reactivity of the alkene: Alkenes are more reactive than alkanes due to the presence of a π-bond, which is electron-rich and can interact with electrophiles. However, in this case, the halogenation reaction is selective, and the alkene does not react as expected.

Consider the reaction conditions: Halogenation of alkanes typically occurs via a free radical mechanism, which involves initiation, propagation, and termination steps. The presence of light or heat is often required to generate radicals. The regioselectivity of the reaction is influenced by the stability of the intermediate radicals formed.

Rationalize the formation of product A: The regioselectivity of the reaction can be explained by the stability of the radical intermediate. When the halogen radical abstracts a hydrogen atom, it preferentially forms the most stable radical. In this case, the radical formed adjacent to the alkene is stabilized by resonance with the π-bond of the alkene, making it the favored intermediate.

Conclude why A is the only product: The formation of the stabilized radical intermediate leads to the selective halogenation at the position adjacent to the alkene. This regioselectivity is a result of the interplay between radical stability and the electronic effects of the alkene. The alkene itself does not react under these conditions because the reaction mechanism is specific to free radical halogenation of the alkane portion of the molecule.

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Key Concepts

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

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In the context of halogenation of alkanes and alkenes, regioselectivity is influenced by the stability of intermediates formed during the reaction, such as carbocations. Understanding how these intermediates stabilize can help predict which product will be favored.

Halogenation Mechanism

Halogenation is a reaction where halogens (like Cl or Br) are added to alkanes or alkenes. The mechanism typically involves the formation of a radical or carbocation intermediate, depending on whether the substrate is an alkane or alkene. The presence of an alkene can alter the pathway and stability of these intermediates, affecting the final product distribution.

Stability of Intermediates

The stability of reaction intermediates, such as carbocations or radicals, plays a crucial role in determining the outcome of organic reactions. More stable intermediates are formed preferentially, leading to specific products. In the case of halogenation in the presence of alkenes, the stability of the formed intermediates can explain why one product is favored over others, as seen in the formation of product A.

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Related Practice

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:

Multiple Choice

Predict the major, organic product of the following reaction.

Multiple Choice

Which reaction is best suited to form the following product?

Textbook Question

The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products:

a. Give a mechanism for this reaction, showing how the two products arise as a consequence of the resonance-stabilized intermediate.

Textbook Question

The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products:

b. The bromination of cyclohexene using NBS gives only one major product, as shown on the previous page. Explain why there is no second product from an allylic shift.

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