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

Pericyclic Reaction

Organic Chemistry

16. Conjugated Systems

Pericyclic Reaction

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8:32 minutes

Problem 20a

Textbook Question

Show that the [4 + 4] cycloaddition of two butadiene molecules to give cycloocta-1,5-diene is thermally forbidden but photochemically allowed.

Verified step by step guidance

Step 1: Recall the Woodward-Hoffmann rules for pericyclic reactions. These rules state that the symmetry of molecular orbitals determines whether a pericyclic reaction is thermally allowed or forbidden. Specifically, for cycloadditions, the reaction is governed by the conservation of orbital symmetry.

Step 2: Analyze the [4 + 4] cycloaddition. Each butadiene molecule contributes 4 π-electrons, so the total number of π-electrons involved in the reaction is 8. This makes it a π-electron system with 4n electrons, where n = 2.

Step 3: Apply the Woodward-Hoffmann rules for thermal reactions. For a thermal cycloaddition to be allowed, the reaction must proceed via a suprafacial-suprafacial interaction of the π-electrons. However, in a [4 + 4] cycloaddition, the symmetry of the molecular orbitals does not allow for a suprafacial-suprafacial interaction in the ground state. Therefore, the reaction is thermally forbidden.

Step 4: Consider the photochemical conditions. Under photochemical conditions, one of the butadiene molecules is excited to a higher energy state, altering the symmetry of its molecular orbitals. This allows for a suprafacial-suprafacial interaction of the π-electrons, making the reaction photochemically allowed.

Step 5: Conclude that the [4 + 4] cycloaddition of two butadiene molecules to form cycloocta-1,5-diene is thermally forbidden due to orbital symmetry constraints but becomes allowed under photochemical conditions due to the altered orbital symmetry in the excited state.

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Key Concepts

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

Cycloaddition Reactions

Cycloaddition reactions involve the joining of two or more unsaturated molecules to form a cyclic structure. In the case of [4 + 4] cycloaddition, two butadiene molecules combine to create cycloocta-1,5-diene. Understanding the mechanism and stereochemistry of these reactions is crucial for predicting their feasibility under different conditions.

Woodward-Hoffmann Rules

The Woodward-Hoffmann rules provide a framework for predicting the outcomes of pericyclic reactions based on their symmetry properties. These rules indicate whether a reaction is allowed or forbidden under thermal or photochemical conditions. For the [4 + 4] cycloaddition, the rules suggest that the reaction is thermally forbidden due to symmetry considerations but can proceed when excited states are involved in photochemical conditions.

Thermal vs. Photochemical Conditions

Thermal conditions refer to reactions occurring at standard temperatures, while photochemical conditions involve the absorption of light to promote reactions. In the context of cycloaddition, the energy provided by light can facilitate the reaction by allowing the system to overcome symmetry constraints that would otherwise render the reaction forbidden under thermal conditions.

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