Cycloadditions: the Diels-Alder reaction

Maitland Jones §11.15 and §21.3

These pages discuss one particular aspect of the Diels-Alder reaction: the use of orbital symmetry to explain the relative ease with which these reactions occur.

As in so many other reactions, the interaction between two reagents involves the highest occupied orbital (HOMO) of one reagent, and the lowest unoccupied orbital (LUMO) of the other. Interaction lowers the energy of the occupied orbital, which lowers the energy of the system. At the same time the unoccupied orbital is destabilized, which has no effect on the overall electron energy.

The HOMO/LUMO interaction depends on two factors:
1. There should be sufficient spatial overlap between the orbitals involved; the more the better.
If the interaction is small, the 'new' orbitals will closely resemble the original mo's.
The overlap is controlled by distance (of course), and by symmetry.

2. Interaction is higher if the energy difference between the HOMO and LUMO is smaller.
That's why a reaction proceeds faster if the HOMO energy is raised (by electron donating substituents), or the LUMO energy decreased (by electron withdrawing substituents).
The new orbitals will become more and more '50/50' mixtures of the original ones.

Let us consider the simple model reaction, between butadiene and ethylene. A "chimed" animation can be viewed separately.
Ethylene has two pi orbitals (lowest one occupied), and butadiene has four (lowest two occupied).

MO's of butadiene and of ethylene, energy increasing from left to right
(click on orbital to view vrml2 file)
ethylene -->

HOMO LUMO
butadiene -->

HOMO-1 HOMO LUMO LUMO+1

**MO's of butadiene and of ethylene, energy increasing from left to right**
(click on orbital to view vrml2 file)
ethylene -->
	HOMO	LUMO
butadiene -->
	HOMO-1	HOMO	LUMO	LUMO+1

Now let's consider the homo/lumo combinations.

As in so many other reactions, the interaction between two reagents involves the highest occupied orbital (HOMO) of one reagent, and the lowest unoccupied orbital (LUMO) of the other. Interaction lowers the energy of the occupied orbital, which lowers the energy of the system. At the same time the unoccupied orbital is destabilized, which has no effect on the overall electron energy.
The HOMO/LUMO interaction depends on two factors: 1. There should be sufficient spatial overlap between the orbitals involved; the more the better. If the interaction is small, the 'new' orbitals will closely resemble the original mo's. The overlap is controlled by distance (of course), and by symmetry.
2. Interaction is higher if the energy difference between the HOMO and LUMO is smaller. That's why a reaction proceeds faster if the HOMO energy is raised (by electron donating substituents), or the LUMO energy decreased (by electron withdrawing substituents). The new orbitals will become more and more '50/50' mixtures of the original ones.