(a) Draw the molecular orbitals for the cyclopropenyl case. H H H (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?

Cycloppropeen is an organic three-membered cyclic ring compound with a molecular formula C₃H₄.It can exist as a cation which is an aromatic compound whereas cyclopropenyl anion is an antiaromatic compound depending upon the electrons.

(a) The cylopropenyl system has three p orbitals so there must be three molecular orbitals. The three molecular orbitals are

(b) The polygon rule states that the molecular orbital energy diagram if a regular completely conjugated cyclic system has the same polygon shape as the compound with one vertex at the bottom. The non-bonding line passes through the center of the polygon. The energy diagram for the cyclopropenyl molecular orbital is represented as:

Representation of bonding, nonbonding antibonding in MO diagram of cyclopropenyl

(c)

The cylopropenyl cation has two pi electrons. These two pi electrons are filled in the bonding molecular orbital. According to the Huckel rule, cylopropenyl cation is an aromatic compound. It is a 4n+2 system.

n = 2

4(0) + 2 = 2

The cyclopropenyl anion has four pi electrons. These four pi electrons are filled in bonding molecular orbital and two degenerate antibonding molecular orbitals. According to the Huckel rule cyclopropenyl anion is an antiaromatic compound. It is a 4n system.

n = 1

4 (1) = 4