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

19. Reactions of Aromatics: EAS and Beyond

Side-Chain Halogenation

Organic Chemistry

19. Reactions of Aromatics: EAS and Beyond

Side-Chain Halogenation

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Problem 62d

Textbook Question

A halogenation intended to make compound A formed B instead.

(d) Without looking it up, would you expect C–H_a or C–H_b to have the lower bond-dissociation energy?

Verified step by step guidance

Identify the type of reaction: The image shows a halogenation reaction using Br2 and heat, which suggests a radical halogenation process.

Analyze the structure of compound A and B: Compound A has the bromine attached to the secondary carbon (C-Ha), while compound B has the bromine attached to the tertiary carbon (C-Hb).

Consider the stability of radicals: In radical halogenation, the stability of the radical intermediate is crucial. Tertiary radicals are more stable than secondary radicals due to hyperconjugation and inductive effects.

Predict the bond-dissociation energy: The bond-dissociation energy is lower for the bond that forms the more stable radical. Therefore, the C-Hb bond, which forms a tertiary radical, is expected to have a lower bond-dissociation energy compared to the C-Ha bond.

Conclude the reasoning: Since compound B is formed, it indicates that the reaction favors the formation of the more stable tertiary radical, which aligns with the expectation that C-Hb has a lower bond-dissociation energy.

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Key Concepts

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

Bond Dissociation Energy

Bond dissociation energy (BDE) is the energy required to break a bond in a molecule, resulting in the separation of atoms. In organic chemistry, BDE is crucial for understanding reaction mechanisms, particularly in radical reactions. Lower BDE indicates a weaker bond, which is more easily broken, influencing the site of halogenation in a molecule.

Radical Halogenation

Radical halogenation involves the substitution of a hydrogen atom in a hydrocarbon with a halogen atom, typically through a radical mechanism. This process is influenced by the stability of the resulting radical, with tertiary radicals being more stable than secondary or primary ones. The stability of the radical formed dictates the preferred site of halogenation.

Radical Stability

Radical stability is determined by the ability of a radical to delocalize its unpaired electron, often through hyperconjugation or resonance. Tertiary radicals are more stable than secondary or primary radicals due to greater hyperconjugation and electron-donating effects from surrounding alkyl groups. This stability influences the outcome of radical reactions, such as halogenation.

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Related Practice

Textbook Question

Predict the product of the following benzylic bromination reactions.

(b)

Textbook Question

Show how you would convert (in one or two steps) 1-phenylpropane to the three products shown below. In each case, explain what unwanted reactions might produce undesirable impurities in the product.

Textbook Question

The solvent tetrahydrofuran (THF) is often sold with a small amount of BHT added. Provide a mechanism that explains why this might be so.

Textbook Question

A halogenation intended to make compound A formed B instead.

(b) Suggest a mechanism that rationalizes the formation of B.

Textbook Question

In the second propagation step in the bromination of toluene, Br₂ is only attacked by a radical on the substituent carbon. Why?

Textbook Question

For each compound, predict the major product of free-radical bromination. Remember that bromination is highly selective, and only the most stable radical will be formed.

(e)

Textbook Question

(c) Based on what you know about the relative stabilities of alkyl radicals and benzylic radicals, predict the product of addition of HBr to 1-phenylpropene in the presence of a free-radical initiator.

(d) Propose a mechanism for this reaction.

Textbook Question

Suggest reagents and reaction conditions that would result in synthesis of the following bromoalkanes.

(c)

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