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

HOMO LUMO

Organic Chemistry

16. Conjugated Systems

HOMO LUMO

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8:04 minutes

Problem 25

Textbook Question

Identify the HOMO and LUMO of the allylic cation and the allylic anion shown in Figure 21.22.
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Verified step by step guidance

Step 1: Understand the concepts of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital). HOMO represents the molecular orbital containing the highest energy electrons, while LUMO represents the molecular orbital that is ready to accept electrons.

Step 2: Recall that allylic systems involve conjugation, where π-electrons are delocalized over three atoms. This delocalization creates molecular orbitals with different energy levels.

Step 3: For the allylic cation, note that it has one fewer electron in the π-system compared to the neutral allylic species. This means the HOMO will be the highest-energy π-orbital that is occupied, and the LUMO will be the next higher-energy π-orbital that is unoccupied.

Step 4: For the allylic anion, note that it has one additional electron in the π-system compared to the neutral allylic species. This means the HOMO will be the highest-energy π-orbital that is occupied, and the LUMO will be the next higher-energy π-orbital that is unoccupied.

Step 5: To identify the specific molecular orbitals, analyze the energy diagram of the allylic system (Figure 21.22). The allylic cation will have a lower HOMO energy due to the electron deficiency, while the allylic anion will have a higher HOMO energy due to the extra electron. The LUMO for both species will depend on the next available unoccupied orbital in the π-system.

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Key Concepts

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

HOMO and LUMO

HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) are fundamental concepts in molecular orbital theory. The HOMO is the highest energy orbital that contains electrons, while the LUMO is the lowest energy orbital that is empty. The energy gap between these two orbitals is crucial for understanding the reactivity and stability of molecules, as it influences how they interact with other species.

Allylic Cation and Anion

An allylic cation is a positively charged species where the positive charge is located on a carbon atom adjacent to a double bond, while an allylic anion has a negative charge on a similar carbon atom. These species are important in organic chemistry due to their unique stability and reactivity patterns, which arise from resonance stabilization. Understanding their structure helps in predicting their behavior in chemical reactions.

Resonance

Resonance is a concept used to describe the delocalization of electrons in molecules where the electron distribution cannot be represented by a single Lewis structure. In the case of allylic cations and anions, resonance allows for the charge to be spread over multiple atoms, enhancing stability. This delocalization is key to understanding the properties and reactivity of these intermediates in organic reactions.

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