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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

16. Conjugated Systems

Conjugated Hydrohalogenation (1,2 vs 1,4 addition)

Organic Chemistry

16. Conjugated Systems

Conjugated Hydrohalogenation (1,2 vs 1,4 addition)

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2:28 minutes

Problem 7c

Textbook Question

Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.
c. cyclopenta-1,3-diene + Br₂ → 3,4-dibromocyclopent-1-ene + 3,5-dibromocyclopent-1-ene

Verified step by step guidance

Identify the type of reaction: This reaction involves the addition of bromine (Br₂) to cyclopenta-1,3-diene, which is a conjugated diene. The reaction proceeds via an electrophilic addition mechanism, and the observed products suggest that both 1,2- and 1,4-addition occur.

Step 1: Activation of bromine molecule: The bromine molecule (Br₂) becomes polarized as it approaches the electron-rich π-system of cyclopenta-1,3-diene. This polarization creates a partial positive charge on one bromine atom, making it electrophilic.

Step 2: Formation of the bromonium ion intermediate: The π-electrons of one of the double bonds in cyclopenta-1,3-diene attack the electrophilic bromine atom, leading to the formation of a cyclic bromonium ion intermediate. This intermediate is stabilized by the conjugated system of the diene.

Step 3: Nucleophilic attack by bromide ion: The bromide ion (Br⁻), generated in the previous step, acts as a nucleophile and attacks the bromonium ion. Depending on the site of attack, two possible products can form: (a) 1,2-addition product (3,4-dibromocyclopent-1-ene) or (b) 1,4-addition product (3,5-dibromocyclopent-1-ene).

Step 4: Analyze the product distribution: The observed mixture of products arises because the bromide ion can attack at different positions of the bromonium ion intermediate. The relative stability of the products and the reaction conditions (e.g., temperature) influence the ratio of 1,2- and 1,4-addition products.

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Key Concepts

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

Electrophilic Addition Reactions

Electrophilic addition reactions involve the addition of an electrophile to a nucleophile, typically across a double bond. In the case of cyclopenta-1,3-diene reacting with Br2, the diene acts as a nucleophile, attacking the bromine molecule, which is an electrophile. This process leads to the formation of a cyclic bromonium ion intermediate, which is crucial for understanding the subsequent product formation.

Stability of Reaction Intermediates

The stability of reaction intermediates, such as carbocations or bromonium ions, significantly influences the outcome of organic reactions. In this reaction, the formation of a bromonium ion allows for the rearrangement and stabilization of the intermediate, leading to different product distributions. The stability of these intermediates can dictate the regioselectivity of the final products, such as 3,4-dibromocyclopent-1-ene and 3,5-dibromocyclopent-1-ene.

Regioselectivity in Electrophilic Additions

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In the reaction of cyclopenta-1,3-diene with Br2, the formation of two different dibrominated products arises from the regioselective attack of the bromide ion on the bromonium ion intermediate. Understanding the factors that influence regioselectivity, such as sterics and electronic effects, is essential for predicting the product distribution in this reaction.

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