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

18. Aromaticity

Frost Circle

Organic Chemistry

18. Aromaticity

Frost Circle

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3:27 minutes

Problem 55b

Textbook Question

Following the instructions for drawing the energy levels of the molecular orbitals for the compounds shown in [Figure 8.17], draw the energy levels of the molecular orbitals for the cycloheptatrienyl cation. For each compound, show the distribution of the $\pi$ electrons. Which of the compounds are aromatic?

Verified step by step guidance

Step 1: Begin by identifying the molecular structure of the cyclopropenyl cation. The cyclopropenyl cation is a three-membered ring with one positive charge and two π-electrons. This is important because the number of π-electrons will determine its aromaticity based on Hückel's rule.

Step 2: Apply Hückel's rule for aromaticity. Hückel's rule states that a compound is aromatic if it has a planar, cyclic structure with (4n + 2) π-electrons, where n is a non-negative integer. For the cyclopropenyl cation, count the π-electrons (2 π-electrons in this case) and check if they satisfy the (4n + 2) rule.

Step 3: Draw the molecular orbital (MO) diagram for the cyclopropenyl cation. Start by noting that the cyclopropenyl cation has three atomic p orbitals (one from each carbon atom) that combine to form three molecular orbitals: one bonding orbital, one non-bonding orbital, and one anti-bonding orbital. Arrange these orbitals in terms of energy levels, with the bonding orbital being the lowest in energy, the non-bonding orbital in the middle, and the anti-bonding orbital being the highest.

Step 4: Populate the molecular orbitals with the π-electrons. Since the cyclopropenyl cation has 2 π-electrons, place them in the lowest energy bonding orbital according to the Aufbau principle (fill lower energy orbitals first). This will leave the non-bonding and anti-bonding orbitals unoccupied.

Step 5: Determine the aromaticity of the cyclopropenyl cation. Since the molecule is cyclic, planar, and has 2 π-electrons (which satisfies the (4n + 2) rule with n = 0), the cyclopropenyl cation is aromatic. Confirm this by noting the delocalization of π-electrons across the ring, which contributes to its stability.

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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 linear combination of atomic orbitals (LCAO) and describe the behavior of electrons in a molecule. In the context of the cyclopropenyl cation, understanding how these orbitals are filled and how they interact is crucial for determining the stability and reactivity of the compound. The arrangement of electrons in these orbitals influences the overall energy levels and can indicate whether a molecule is aromatic.

Aromaticity

Aromaticity is a property of cyclic compounds that exhibit enhanced stability due to the delocalization of π electrons across the ring structure. For a compound to be considered aromatic, it must satisfy Hückel's rule, which states that it should have a planar structure, be cyclic, and contain a total of 4n + 2 π electrons, where n is a non-negative integer. Identifying whether the cyclopropenyl cation is aromatic involves analyzing its electron count and structure.

Electron Distribution

Electron distribution refers to how electrons are arranged in molecular orbitals and how they contribute to the overall electronic structure of a molecule. In the case of the cyclopropenyl cation, understanding the distribution of p electrons is essential for visualizing the bonding and stability of the compound. This distribution can be represented in energy level diagrams, which help in assessing the compound's reactivity and aromatic character.

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Use a Frost circle diagram to construct the molecular orbital diagram for the molecules shown. Would you expect them to be aromatic or antiaromatic?

(b)

Textbook Question

Using a Frost circle, a student drew the molecular orbital picture shown for the cyclopentadienyl anion, determining it to be antiaromatic. Do you agree with this conclusion? Why or why not?

Textbook Question

Following the instructions for drawing the energy levels of the molecular orbitals for the compounds shown in [Figure 8.17], draw the energy levels of the molecular orbitals for the cycloheptatrienyl cation. For each compound, show the distribution of the $π$ electrons. Which of the compounds are aromatic?

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Open Question

Draw the inscribed polygon for the following molecule.

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The average C―C single bond length is 1.53 Å, and the average C=C double bond length is 1.31 Å. All of the C―C bonds in benzene are the same length (1.42 Å). Explain.

Textbook Question

An incomplete Frost diagram for a seven-membered ring is shown. Include the necessary electrons to make it represent an aromatic molecule. Then the draw the structure to which it would correspond.

Textbook Question

Use a Frost circle diagram to construct the molecular orbital diagram for the molecules shown. Would you expect them to be aromatic or antiaromatic?

(a)

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