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

Free Radical Halogenation

Organic Chemistry

11. Radical Reactions

Free Radical Halogenation

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1:57 minutes

Problem 50

Textbook Question

Explain why the rate of bromination of methane decreases if HBr is added to the reaction mixture.

Verified step by step guidance

Identify the reaction mechanism for the bromination of methane, which is a free radical chain reaction involving initiation, propagation, and termination steps.

Understand that the initiation step involves the homolytic cleavage of Br2 to form two bromine radicals, which is crucial for the propagation of the chain reaction.

Recognize that the addition of HBr introduces additional bromine atoms in a non-radical form, which can react with the bromine radicals, effectively reducing their concentration.

Consider how the presence of HBr can lead to a termination step where bromine radicals react with HBr to form non-radical products, thus decreasing the number of radicals available for the propagation steps.

Conclude that the decrease in the concentration of bromine radicals due to the presence of HBr slows down the overall rate of the bromination reaction, as fewer radicals are available to sustain the chain reaction.

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Key Concepts

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

Bromination Mechanism

Bromination of methane typically occurs via a free radical mechanism, where bromine radicals (Br·) abstract hydrogen atoms from methane (CH4) to form bromomethane (CH3Br). This process involves initiation, propagation, and termination steps, with the rate of reaction heavily dependent on the concentration of bromine radicals in the mixture.

Effect of HBr on Radical Concentration

The addition of HBr introduces bromide ions (Br-) into the reaction mixture, which can react with bromine radicals to form HBr and regenerate bromine. This reaction effectively reduces the concentration of free bromine radicals available for the bromination of methane, thereby decreasing the overall rate of the bromination process.

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system shifts to counteract the change. In this case, the addition of HBr shifts the equilibrium of the bromination reaction, favoring the formation of HBr over the production of bromine radicals, thus slowing down the rate of bromination.

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Predict the major products of the following alkane halogenation reactions. [The number of products shown ignores the formation of racemic mixtures.]

(c)

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Cyclizations can be carried out under radical conditions involving Bu₃SnH and a catalytic amount of AIBN. Suggest a mechanism for this reaction. [Pay close attention to the bonds formed and broken in developing your mechanism. It may be helpful to number the carbons.]

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How many alkyl halides are obtained from monochlorination of the alkanes in Problem 4 if stereoisomers are included?

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How many monochlorination products would be obtained if all stereoisomers are included?

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What is the major product obtained from treating an excess of each of the following compounds with Cl₂ in the presence of ultraviolet light at room temperature? Disregard stereoisomers.

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