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

16. Conjugated Systems

Cumulative Electrocyclic Problems

Organic Chemistry

16. Conjugated Systems

Cumulative Electrocyclic Problems

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8:50 minutes

Problem 43

Textbook Question

Explain why compound A will not undergo a ring-opening reaction under thermal conditions, but compound B will.

Verified step by step guidance

Analyze the structures of compound A and compound B. Compound A is a bicyclic system with a six-membered ring fused to a three-membered ring, while compound B is a bicyclic system with an eight-membered ring fused to a three-membered ring.

Consider the strain in the three-membered ring. Both compounds have a cyclopropane ring, which is highly strained due to its small bond angles (approximately 60° compared to the ideal tetrahedral angle of 109.5°). This strain makes cyclopropane rings susceptible to ring-opening reactions.

Evaluate the strain in the larger rings. Compound A has a six-membered ring, which is relatively stable due to its ability to adopt a chair conformation, minimizing angle and torsional strain. Compound B has an eight-membered ring, which is less stable due to increased angle strain and torsional strain, making it more reactive under thermal conditions.

Consider the thermal conditions. Under heat, ring-opening reactions are more likely to occur if the reaction leads to a decrease in overall strain. For compound A, opening the cyclopropane ring would not significantly reduce strain because the six-membered ring is already stable. For compound B, opening the cyclopropane ring would relieve strain in both the cyclopropane and the eight-membered ring, making the reaction thermodynamically favorable.

Conclude that compound A will not undergo a ring-opening reaction under thermal conditions because the strain relief is minimal, while compound B will undergo the reaction because it leads to significant strain relief in the eight-membered ring.

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Key Concepts

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

Thermal Stability of Cyclic Compounds

The thermal stability of cyclic compounds is influenced by their ring strain and the nature of the substituents. Compounds with lower ring strain, such as those with six-membered rings, are generally more stable and less likely to undergo ring-opening reactions under thermal conditions. In contrast, compounds with higher ring strain, like three- or four-membered rings, are more reactive and can easily open up when heated.

Mechanism of Ring-Opening Reactions

Ring-opening reactions typically involve the breaking of a sigma bond in the ring structure, which can be facilitated by heat or the presence of nucleophiles. The mechanism often depends on the stability of the transition state and the energy required to break the bonds. Compounds that can stabilize the transition state or have favorable sterics or electronics are more likely to undergo these reactions.

Substituent Effects on Reactivity

The presence and nature of substituents on a cyclic compound can significantly affect its reactivity. Electron-withdrawing groups can stabilize positive charges in transition states, while electron-donating groups can destabilize them. This means that compounds with certain substituents may be more prone to ring-opening under thermal conditions, while others may remain stable due to the electronic effects of their substituents.