Diels-Alder Reaction: A Simple Explanation?

[Qube]

Homework Statement

This is my prof's explanation of Diels-Alder ... below is the mechanism and some resonance structures he drew to explain the concerted movement of electrons to the class ...

Homework Equations

Nucleophiles attack electrophiles.

Electrons in HOMOs go into LUMOs.

The Attempt at a Solution

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So my professor's explanation is very interesting for a few reasons:

1) He completely sidesteps all the ramblings about molecular orbitals in the text.

2) His electrostatic explanation is very tempting based on Occam's razor alone but ethene is a gas at STP. If the dipolar resonance structure were a big contributor then wouldn't ethene instead be a liquid?

3) The book has a section on stereoselectivity and Diels-Alder reactions. How can pure electrostatics explain stereoselectivity?

So am I crazy or what for thinking that the guy's electrostatic explanation sounds like a story of unicorns in the park?

 

[Bystander]

unicorns in the park?

Are you certain he wasn't discussing "orbital symmetries" with his plus and minus signs?

 

[Qube]

Are you certain he wasn't discussing "orbital symmetries" with his plus and minus signs?

Yes I'm sure he didn't mention orbitals at all.