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

15. Analytical Techniques:IR, NMR, Mass Spect

Purpose of Analytical Techniques

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

Purpose of Analytical Techniques

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Problem 88

Textbook Question

A student obtained two products from the reaction of 1,3-cyclohexadiene with Br₂ (disregarding stereoisomers). His lab partner was surprised when he obtained only one product from the reaction of 1,3-cyclohexadiene with HBr (disregarding stereoisomers). Account for these results.

Verified step by step guidance

Step 1: Recognize that 1,3-cyclohexadiene is a conjugated diene, meaning it has alternating double bonds, which allows for resonance stabilization of intermediates during reactions.

Step 2: Analyze the reaction with Br₂. This reaction proceeds via an electrophilic addition mechanism. The Br₂ molecule acts as an electrophile, and the π-electrons of the diene attack Br₂, forming a bromonium ion intermediate. This intermediate can be attacked by a bromide ion (Br⁻) at two different positions, leading to two possible products: 1,2-addition (kinetic product) and 1,4-addition (thermodynamic product).

Step 3: Analyze the reaction with HBr. In this case, the reaction also proceeds via an electrophilic addition mechanism. The H⁺ from HBr acts as the electrophile, and the π-electrons of the diene attack H⁺, forming a carbocation intermediate. Due to resonance stabilization, the carbocation intermediate is delocalized, but the addition of Br⁻ occurs at the most stable position, leading to only one major product.

Step 4: Compare the two reactions. The reaction with Br₂ allows for two different pathways (1,2- and 1,4-addition) because the bromonium ion intermediate can be attacked at two distinct positions. In contrast, the reaction with HBr leads to a single product because the carbocation intermediate directs the nucleophile (Br⁻) to the most stable position, resulting in regioselectivity.

Step 5: Conclude that the difference in the number of products arises from the nature of the intermediates formed in each reaction. The bromonium ion in the Br₂ reaction allows for multiple attack sites, while the carbocation in the HBr reaction leads to a single, regioselective product.

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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 1,3-cyclohexadiene reacting with Br2, the bromine molecule acts as an electrophile, leading to the formation of two products due to the presence of two double bonds that can react. This reaction is characterized by the formation of a cyclic bromonium ion intermediate, which can lead to different products depending on the pathway taken.

Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX (where X is a halogen) to an alkene, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. In the reaction of 1,3-cyclohexadiene with HBr, this rule explains why only one product is formed, as the addition occurs at the more substituted carbon, leading to a single major product rather than multiple products.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In the case of 1,3-cyclohexadiene reacting with HBr, the regioselectivity is influenced by the stability of the carbocation intermediates formed during the reaction. This leads to the formation of a single product, as the reaction pathway favors the more stable carbocation, resulting in less competition for product formation.

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