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

10. Addition Reactions

Halogenation

Organic Chemistry

10. Addition Reactions

Halogenation

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7:23 minutes

Problem 9a

Textbook Question

In contrast to the addition of Br₂ the addition of HBr [Section 8.3] is not stereospecific. Why?

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The addition of Br₂ to an alkene is stereospecific because it proceeds through a bromonium ion intermediate. This intermediate is a three-membered ring structure that forces the incoming nucleophile (Br⁻) to attack from the opposite side of the ring, leading to anti-addition. This ensures a specific stereochemical outcome.

In contrast, the addition of HBr to an alkene is not stereospecific because it proceeds through a carbocation intermediate. The formation of the carbocation is planar, allowing the nucleophile (Br⁻) to attack from either side of the plane, leading to a mixture of stereoisomers.

The image shows that the addition of Br₂ results in a single stereochemical outcome (anti-addition), while the addition of HBr results in a mixture of stereoisomers due to the non-stereospecific nature of the reaction.

The key difference lies in the intermediates: the bromonium ion in the Br₂ addition restricts the approach of the nucleophile, while the planar carbocation in the HBr addition allows for attack from both sides.

Thus, the addition of HBr is not stereospecific because the planar carbocation intermediate does not impose any stereochemical constraints on the reaction.

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Key Concepts

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

Stereospecificity

Stereospecificity refers to a reaction where the stereochemistry of the reactants influences the stereochemistry of the products. In stereospecific reactions, a specific stereoisomer of a reactant will yield a specific stereoisomer of the product. This is crucial in understanding reactions like the addition of Br₂, where the formation of a cyclic bromonium ion leads to a specific stereochemical outcome.

Electrophilic Addition Reactions

Electrophilic addition reactions involve the addition of an electrophile to a nucleophile, typically across a double bond. In the case of HBr addition, the hydrogen atom acts as the electrophile, while the double bond acts as the nucleophile. Unlike Br₂, which forms a cyclic intermediate leading to stereospecificity, HBr does not form such intermediates, resulting in a non-stereospecific addition.

Reaction Mechanism

The reaction mechanism describes the step-by-step process by which reactants are converted into products. For HBr addition, the mechanism involves the protonation of the alkene followed by nucleophilic attack by bromide. This mechanism does not favor a specific stereochemical pathway, leading to a mixture of products, unlike the more defined pathway seen in Br₂ addition.

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Related Practice

Textbook Question

Predict the product(s) that would result when the alkenes are allowed to react under the following conditions: (i) Br₂ (ii) Cl₂

(f)

Textbook Question

Predict the product of the following haloalkane syntheses.

(a)

Textbook Question

Predict the product(s) of each of the following reactions, making sure to indicate the relative stereochemical outcome. Indicate any racemic mixtures by drawing both enantiomers.

(c)

Textbook Question

In spite of being mechanistically similar to some of the reactions we saw in Chapter 8, rearrangement never occurred here in Chapter 9. Why doesn't rearrangement occur in the following bromination reaction despite the proximity of a more substituted carbon?

Textbook Question

Show how each of the following compounds can be synthesized from an alkene:

Textbook Question

How does the first step in the reaction of propene with Br₂ differ from the first step in the reaction of propene with HBr?

Textbook Question

To understand why Br⁻ adds to a carbon of the bromonium ion rather than to the positively charged bromine, draw the product that would be obtained if Br⁻ did add to bromine.

Textbook Question

What is the product of the addition of I—Cl to 1-butene? (Hint: Chlorine is more electronegative than iodine [Table 1.3].)

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