Experiment: Diels-Alder Reaction
Week 5

Analogous to last week's experiment, again we see that there is an exo and endo assignment to the possible products in the Diels-Alder adducts. Consider the following figure:

Exo assignment is given to the structure that bears the maleic anhydride portion in proximity to the smaller carbon bridge. Conversely, endo assignment is given to the structure with the maleic anhydride closer to the larger carbon bridge.


In the reader and in lecture, you have learned that the HOMO (Highest Occupied Molecular Orbital) of one compound must react with the LUMO (Lowest Unoccupied Molecular Orbital) of another. Here is a brief overview as to how these energy levels are generated:

Each lobe in the figure above corresponds to a p-orbital. Notice how the p-orbitals are arranged in the same conjugated system as in the pi-bonds in cyclopentadiene. Each orbital has two poles (shaded and non-shaded). It does not matter which side you choose to be shaded, JUST AS LONG AS YOU ARE CONSISTENT IN YOUR ASSIGNMENT THROUGHOUT THE GENERATION OF THESE ENERGY LEVELS.

The p-orbitals can be arranged in such a way as to create nodes, or changes in orbital signs. The number of nodes is directly proportional to the relative value of energy levels

Notice that the lowest energy level (energy level 1) has zero nodes (all orbitals are shaded on top and clear on bottom). There are no sign changes (no alternation in shaded to clear orbitals on the same side of the molecule). In the second energy level there is one node as denoted by the dashed line. Take note that the orbitals are aligned such that each double bond (referring to the original molecule) has two of the same orbitals. This partially explains the reason why the next energy level (level 3) is higher in value, but still has one node.

The third energy level has one node, but the distribution of shaded and clear orbitals is in such a way that the two orbitals on the right half of similar sign create a through-space conjugation. This through-space conjugation is much less favorable than a through-bond conjugation as observed in energy levels 1 and 2.

Lastly, energy level four is highest (most unstable) because of its two nodes, where there is no orbital overlap to enable pi-conjugation.

Since there are two pi-bonds in cyclopentadiene, there are four pi-electrons. These pi electrons are filled in from the lowest energy level (maximum of two electrons per energy level). Thus energy level three is the LUMO and energy level two is the HOMO.

Confused? Ask a TA.