Q45E - Organic Chemistry (Mcmurry)

Question

Show the relative energy levels of the seven p molecular orbitals of the cycloheptatrienyl system. Tell which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation.

Step-by-Step Solution

Answer

 

                      Energy level  diagram of cycloheptatrienyl cation, radical, and anion

Step 1: Energy level

The seven molecular orbitals are combined to give seven pi molecular orbitals, with a single lowest energy level orbital and degenerate pairs of the higher energy orbitals.

Step 2: Cycloheptatrienyl cation, radical and anion

In the six pi-electron cations, there are two electrons in ${\mathrm{\psi }}_1$ and also two electrons in ${\mathrm{\psi }}_2$ and ${\mathrm{\psi }}_3$. Thus, the cation doesn’t have any partially filled orbitals, therefore, cycloheptatrienyl cation is aromatic. 

In the remaining two cyclopheptatrienyl radical and cycloheptatrienyl anion, the pi electrons are occupied in the anti-bonding orbital also. So, these two systems are not aromatic compounds.

Energy level diagram of cycloheptatrienyl cation, radical, and anion