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

9. Alkenes and Alkynes

Alkene Stability

Organic Chemistry

9. Alkenes and Alkynes

Alkene Stability

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Problem 51

Textbook Question

Consider the Cope rearrangement, a reaction we describe in Chapter 20.
(a) Using the knowledge we have gained here in Chapter 9, suggest a one-step, concerted mechanism that explains the formation of B from A.
(b) Which side of the reaction would you expect to be favored? Justify your answer.
(c) Which product, A or B, would you expect to be hydrogenated with the more exothermic heat of hydrogenation?

Verified step by step guidance

Step 1: Analyze the Cope rearrangement mechanism. The Cope rearrangement is a [3,3]-sigmatropic reaction, which involves the concerted movement of electrons. In this case, the reaction starts with compound A, where the π-electrons in the double bonds and the σ-electrons in the single bonds rearrange to form compound B. Draw the curved arrows to show the electron movement.

Step 2: Consider the thermodynamic stability of A and B to determine which side of the reaction is favored. Compound B introduces a stereogenic center and a conjugated diene system, which increases stability due to conjugation. Compare the stability of the conjugated diene in B versus the isolated double bonds in A.

Step 3: Evaluate the heat of hydrogenation for A and B. Hydrogenation is more exothermic for less stable compounds. Since A has isolated double bonds and lacks conjugation, it is less stable than B. Therefore, A would release more heat upon hydrogenation compared to B.

Step 4: Justify the favorability of the reaction direction. The formation of B is favored due to the increased stability from conjugation and the stereogenic center. This aligns with the principle that reactions tend to favor the formation of more stable products.

Step 5: Summarize the mechanism and thermodynamic considerations. The Cope rearrangement proceeds via a concerted mechanism, favoring the formation of B due to its conjugated diene system. A would exhibit a more exothermic heat of hydrogenation due to its lower stability.

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Key Concepts

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

Cope Rearrangement

The Cope rearrangement is a thermal rearrangement of 1,5-hexadienes that involves a concerted mechanism, resulting in the migration of substituents across a double bond. This reaction is characterized by the formation of a new sigma bond and the breaking of an existing sigma bond, all occurring in a single step without intermediates. Understanding this rearrangement is crucial for predicting the products formed from the starting materials in the question.

Concerted Mechanism

A concerted mechanism refers to a reaction pathway where bond breaking and bond forming occur simultaneously in a single step, without the formation of intermediates. This type of mechanism is essential in understanding the Cope rearrangement, as it allows for the prediction of product formation based on the alignment and overlap of orbitals during the transition state. Recognizing the concerted nature of the reaction helps in visualizing how the reactants transform into products.

Heat of Hydrogenation

The heat of hydrogenation is the amount of energy released when a compound is hydrogenated, which can be used to assess the stability of alkenes. More stable alkenes have lower heats of hydrogenation because they are less reactive and require less energy to convert to alkanes. In the context of the question, comparing the heats of hydrogenation for products A and B will help determine which product is more stable and thus favored in the reaction.

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Related Practice

Textbook Question

Conjugated dienes, molecules containing two alkenes separated by one single bond, are discussed in detail in Chapter 21.

(b) How might you account for this difference in stability?

Textbook Question

Bromination of buta-1,3-diene with a single equivalent of Br₂ can give either of two products. (a) Which of these products (A or B) would you predict to be more stable? Justify your answer.

Textbook Question

Rank each group of compounds in order of increasing heat of hydrogenation.

(b)

Textbook Question

Rank each group of compounds in order of increasing heat of hydrogenation.

(a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene

Textbook Question

Rank the following alkenes in order of stability (1 = most stable; 5 = least stable). Explain your order.

Textbook Question

The same alkane is obtained from the catalytic hydrogenation of both alkene A and alkene B. The heat of hydrogenation of alkene A is 29.8 kcal/mol, and the heat of hydrogenation of alkene B is 31.4 kcal/mol. Which alkene is more stable?

Textbook Question

a. Which is the most stable: 3,4-dimethyl-2-hexene, 2,3-dimethyl-2-hexene, or 4,5-dimethyl-2-hexene?

b. Which compound has the largest heat of hydrogenation?

c. Which compound has the smallest heat of hydrogenation?

Textbook Question

Of the compounds you named in Problem 45:

a. Which is the most stable?

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