Chapter 14, Problem 14.14P

Interpretation Introduction

Interpretation:

Based on the characteristics of the MO diagram of cyclobutadienyl dication, it is to be determined if it is aromatic, antiaromatic, or nonaromatic.

Concept introduction:

To derive the relative energies of pi molecular orbitals, a Frost method is used. This method is applicable to any species with a fully conjugated, cyclic pi system. This method involves drawing a polygon that represents the cyclic compound’s line structure, with one of the vertices pointing directly upwards. Each of the four vertices then represents the energy of a $\text{π}$ MO ${\text{(π}}_{\text{1}}{\text{, π}}_{\text{2}}{\text{, π}}_{\text{3}}{\text{, and π}}_{\text{4}}^{\text{*}})$, and the center of the square represents the energy of each unhybridized p atomic orbitals.

Aromatic compounds have cyclic $\text{π}$ systems that are unusually stable.

Antiaromatic compounds have cyclic $\text{π}$ systems that are unusually unstable.

Nonaromatic compounds are neither unusually stable nor unusually unstable.