Chapter 24, Problem 24.61P

Interpretation Introduction

(a)

Interpretation:

The HOMO of one reactant and LUMO of the other reactant in the given reaction are to be drawn assuming both are in their ground states.

Concept introduction:

The bonding or antibonding character of an MO is determined by the number of constructive and destructive interactions. When the number of constructive (same phase) interactions exceed the number of destructive interactions, the MO is bonding. If the number of destructive interactions is larger, the MO is antibonding.

The highest energy bonding MO is the last one that contains electrons and is called the Highest Energy Molecular orbital (HOMO). The antibonding MO immediately above it in is the Lowest Energy Unoccupied Molecular Orbital, called LUMO.

Interpretation Introduction

(b)

Interpretation:

The HOMO-LUMO interaction between the two reactants is to be drawn and if it will result in a thermally allowed or forbidden reaction is to be determined.

Concept introduction:

Diels-Alder reaction is a concerted reaction between a diene and a dienophile that produces a cyclic compound. It replaces two $\text{π}$ bonds with sigma bonds. The flow of all $\text{π}$ involved is in the same direction.

In terms of MO theory, the reaction requires that the overlapping orbitals of the two molecules be in phase so that constructive interference occurs.

The electron flow starts with the diene donating a pair of $\text{π}$ electrons to the dienophile. This means the HOMO of the diene overlaps with the LUMO of the dienophile. The interactions between the p orbitals of the carbon atoms result in the formation of an equal number of MOs, half of them bonding and half antibonding orbitals.