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

Orbital Diagram:5-atoms- Allylic Ions

Organic Chemistry

16. Conjugated Systems

Orbital Diagram:5-atoms- Allylic Ions

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6:28 minutes

Problem 37f

Textbook Question

The pentadienyl radical, H₂C=CH–CH=CH–CH₂•, has its unpaired electron delocalized over three carbon atoms.
f. Show how your molecular orbital picture agrees with the resonance picture showing delocalization of the unpaired electron onto three carbon atoms.

Verified step by step guidance

Begin by identifying the structure of the pentadienyl radical, H₂C=CH-CH=CH-CH₂·. Note that the unpaired electron is located on the terminal carbon atom (CH₂·). This radical is conjugated because it contains alternating double bonds and a single electron, allowing for delocalization.

Draw the resonance structures for the pentadienyl radical. Start by moving the π-electrons of the double bonds to adjacent positions while shifting the unpaired electron. This will result in three resonance structures where the unpaired electron is delocalized over the first, third, and fifth carbon atoms.

Introduce the concept of molecular orbitals (MOs). In the pentadienyl radical, there are five p orbitals (one on each carbon atom) that overlap to form a conjugated π-system. These p orbitals combine to form five molecular orbitals: three bonding, one non-bonding, and one anti-bonding.

Explain that the unpaired electron occupies the non-bonding molecular orbital. This orbital has equal contributions from the p orbitals on the first, third, and fifth carbon atoms, which aligns with the resonance structures showing delocalization of the unpaired electron over these three carbons.

Conclude by emphasizing that the molecular orbital picture supports the resonance picture. The delocalization of the unpaired electron over three carbon atoms is a result of the conjugated π-system and the non-bonding molecular orbital, which is consistent with the resonance structures drawn earlier.

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Key Concepts

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

Molecular Orbitals

Molecular orbitals (MOs) are formed by the combination of atomic orbitals when atoms bond together. In the case of the pentadienyl radical, the unpaired electron can occupy a molecular orbital that is delocalized over multiple carbon atoms, allowing for resonance stabilization. Understanding how these orbitals overlap helps visualize the distribution of the unpaired electron across the molecule.

Resonance Structures

Resonance structures are different Lewis structures that represent the same molecule, illustrating how electrons are distributed. For the pentadienyl radical, resonance structures show the delocalization of the unpaired electron across three carbon atoms, indicating that the actual structure is a hybrid of these forms. This concept is crucial for understanding the stability and reactivity of radicals.

Delocalization of Electrons

Delocalization refers to the spreading of electrons across multiple atoms rather than being localized between two. In the pentadienyl radical, the unpaired electron is delocalized over three carbon atoms, which lowers the energy of the system and increases stability. This phenomenon is key to understanding the behavior of radicals and their reactivity in organic chemistry.

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Related Practice

Textbook Question

Answer the following questions for the MOs of 1,3-butadiene:

a. Which are $π$ bonding MOs, and which are $π$ * antibonding MOs?

b. Which MOs are symmetric, and which are antisymmetric?

c. Which MO is the HOMO and which is the LUMO in the ground state?

Open Question

Predict the molecular orbitals and identify the HOMO and LUMO orbitals of the following cation.

Textbook Question

The pentadienyl radical, H₂C=CH–CH=CH–CH₂•, has its unpaired electron delocalized over three carbon atoms.

a. Use resonance forms to show which three carbon atoms bear the unpaired electron.

Textbook Question

The pentadienyl radical, H₂C=CH–CH=CH–CH₂•, has its unpaired electron delocalized over three carbon atoms.

b. How many MOs are there in the molecular orbital picture of the pentadienyl radical?

c. How many nodes are there in the lowest-energy MO of the pentadienyl system? How many in the highest-energy MO?

d. Draw the MOs of the pentadienyl system in order of increasing energy

Textbook Question

The pentadienyl radical, H₂C=CH–CH=CH–CH₂•, has its unpaired electron delocalized over three carbon atoms.

g. Remove the highest-energy electron from the pentadienyl radical to give the pentadienyl cation. Which carbon atoms share the positive charge? Does this picture agree with the resonance picture?

Textbook Question

The pentadienyl radical, H₂C=CH–CH=CH–CH₂•, has its unpaired electron delocalized over three carbon atoms.

h. Add an electron to the pentadienyl radical to give the pentadienyl anion. Which carbon atoms share the negative charge? Does this picture agree with the resonance picture?

Textbook Question

Answer the following questions for the MOs of 1,3-butadiene:

a. Which are $π$ bonding MOs, and which are $π$ * antibonding MOs?

b. Which MOs are symmetric, and which are antisymmetric?

c. Which MO is the HOMO and which is the LUMO in the ground state?

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