(a) Draw the molecular orbitals for the cyclopropenyl case.
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(Because there are three p orbitals, there must be three MOs:
one all-bonding MO and one degenerate pair of MOs.)
(b) Draw an energy diagram for the cyclopropenyl MOs.
(The polygon rule is helpful.)
Label each MO as bonding, nonbonding, or antibonding,
and add the nonbonding line. Notice that it goes through
the approximate average of the MOs.
(c) Add electrons to your energy diagram to show
the configuration of the cyclopropenyl cation and
the cyclopropenyl anion. Which is aromatic and
which is antiaromatic?

Verified step by step guidance

1. Identify the number of p orbitals in cyclopropenyl, which is three, and understand that this leads to the formation of three molecular orbitals (MOs).
2. Draw the molecular orbitals: start with the lowest energy MO, which is the all-bonding MO where all p orbitals are in phase. Then, draw the degenerate pair of MOs, which are higher in energy and have nodes.
3. Use the polygon rule to draw an energy diagram: inscribe a triangle (since cyclopropenyl is a three-membered ring) in a circle, with the vertices representing the energy levels of the MOs.
4. Label the MOs in the energy diagram: the lowest energy MO is bonding, the degenerate pair is antibonding, and the nonbonding line is drawn through the center of the diagram.
5. Add electrons to the energy diagram: for the cyclopropenyl cation, add two electrons to the lowest MO; for the cyclopropenyl anion, add four electrons. Determine aromaticity based on the electron configuration and Hückel's rule.

Key Concepts

Molecular Orbitals (MOs)

Molecular orbitals are formed by the combination of atomic orbitals when atoms bond together. In the case of cyclopropenyl, three p orbitals combine to create three MOs: one bonding orbital and a degenerate pair of nonbonding orbitals. Understanding how these orbitals are formed and their energy levels is crucial for analyzing the stability and reactivity of the molecule.

Energy Diagram

An energy diagram visually represents the relative energy levels of molecular orbitals. It helps in identifying bonding, nonbonding, and antibonding orbitals. The polygon rule assists in determining the arrangement of these orbitals, and the nonbonding line indicates the average energy level of the nonbonding orbitals, which is essential for understanding electron configurations in different ionic states.

Aromaticity and Antiaromaticity

Aromaticity refers to the enhanced stability of cyclic compounds with a planar structure and a continuous overlap of p orbitals, following Hückel's rule (4n+2 π electrons). In contrast, antiaromatic compounds have 4n π electrons, leading to instability. Identifying whether the cyclopropenyl cation or anion is aromatic or antiaromatic is key to predicting their chemical behavior and stability.