Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions

Problem 21

Chapter 22, Problem 21

Draw a Frost circle for the cyclopropenyl anion and compare it to the Frost circle for the cyclopropenyl cation. What has changed?

Verified step by step guidance

To draw a Frost circle for the cyclopropenyl anion, start by drawing a circle with a triangle inscribed inside it, where each vertex of the triangle represents a carbon atom in the cyclopropenyl anion.

Position the triangle such that one of its vertices points downward, touching the bottom of the circle. This represents the energy levels of the molecular orbitals.

For the cyclopropenyl anion, which has 4 π electrons, fill the molecular orbitals starting from the lowest energy level. Each line in the Frost circle represents a molecular orbital, and each orbital can hold 2 electrons.

Now, compare this to the Frost circle for the cyclopropenyl cation, which has 2 π electrons. The cation's Frost circle will have the same triangular arrangement, but only the lowest energy level will be filled with 2 electrons.

The key difference between the anion and cation Frost circles is the number of electrons and the resulting filling of the molecular orbitals. The anion has an extra electron, which fills the next available orbital, altering the electronic configuration and stability of the molecule.

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Key Concepts

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

Frost Circle Method

The Frost Circle method is a graphical tool used to predict the relative energies of molecular orbitals in cyclic conjugated systems. By inscribing a polygon representing the molecule inside a circle, the vertices indicate the energy levels of the orbitals. This method helps visualize the stability and electronic configuration of molecules like cyclopropenyl anion and cation.

Cyclopropenyl Anion and Cation

Cyclopropenyl anion and cation are three-membered ring structures with differing charges. The anion has an extra electron, resulting in a negative charge, while the cation lacks an electron, giving it a positive charge. These charge differences affect the electronic structure and stability, which can be analyzed using Frost circles to compare their molecular orbital energies.

Molecular Orbital Theory

Molecular Orbital Theory explains the behavior of electrons in molecules by describing them as delocalized orbitals spread over the entire molecule. It helps predict the stability and reactivity of molecules by analyzing the filling of these orbitals. In the context of cyclopropenyl anion and cation, this theory aids in understanding how the addition or removal of electrons affects their electronic configuration and stability.

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Related Practice

Textbook Question

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.

(ii) For the aromatic and antiaromatic molecules, solve for n in Hückel’s/Breslow’s rule. For the other molecules, explain which of the rules of aromaticity is being broken.

(d)

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Textbook Question

Name the following benzene derivatives according to IUPAC rules.

(c)

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Textbook Question

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.

(ii) For the aromatic and antiaromatic molecules, solve for n in Hückel’s/Breslow’s rule. For the other molecules, explain which of the rules of aromaticity is being broken.

(b)

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Textbook Question

Which resonance structure, A or B, is most contributing?

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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.

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Textbook Question

Draw a Frost circle for the cyclopentadienyl cation and compare it to the Frost circle for the cyclopentadienyl anion. What has changed?

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