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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3:27 minutes

Problem 25d

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.

Verified step by step guidance

Cyclohexene contains a double bond, which is an area of high electron density. This makes it more reactive toward electrophiles like bromine compared to cyclohexane, which lacks a double bond and is less reactive.

In the presence of light (hv), bromine undergoes homolytic cleavage to form bromine radicals. These radicals initiate a chain reaction with cyclohexene.

The double bond in cyclohexene allows for the formation of a more stable allylic radical during the reaction. This stability arises from resonance delocalization of the unpaired electron over the π-system of the double bond.

Cyclohexane, on the other hand, does not have a double bond and cannot form a resonance-stabilized radical. As a result, its reaction with bromine requires higher energy input, such as heating, to proceed.

The reaction of cyclohexene with bromine is faster because the allylic radical intermediate is stabilized, lowering the activation energy for the reaction and making it more favorable under light-promoted conditions.

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Key Concepts

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

Electrophilic Addition

Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, leading to the formation of a more saturated product. In the case of cyclohexene, the double bond acts as a nucleophile, readily reacting with bromine, an electrophile, to form bromocyclohexene. This reaction is favored due to the stability of the carbocation intermediate formed during the process.

Stability of Alkenes vs. Alkanes

Alkenes, such as cyclohexene, are generally more reactive than alkanes like cyclohexane due to the presence of a carbon-carbon double bond. This double bond is a site of high electron density, making alkenes more susceptible to electrophilic attack. In contrast, alkanes are saturated hydrocarbons with only single bonds, requiring higher energy conditions, such as heat, to initiate reactions with electrophiles like bromine.

Reaction Conditions and Activation Energy

The reaction conditions significantly influence the rate of chemical reactions. Cyclohexene reacts with bromine under mild conditions due to its lower activation energy, allowing the reaction to proceed at room temperature or with light. Conversely, cyclohexane requires heating to provide the necessary energy to overcome the activation barrier for bromination, making it less reactive under standard conditions.

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Related Practice

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 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, 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:

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.

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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.

Key Concepts

Electrophilic Addition

Stability of Alkenes vs. Alkanes

Reaction Conditions and Activation Energy

Watch next

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.