Diels-Alder Reaction and MMO (More Molecular Orbitals!)

Today was a hard day. We spent the first part going over one way to figure out the structures of molecules when give spectral data; I did problem #2 from the ND web site. The due date for the group NMR problems was moved back to NEXT Friday.

We looked at the Diels-Alder reaction -- a way to make cyclohexenes from a diene and a dienophile (which is generally a fancy name for an alkene). Since there is no obvious Lewis acid and Lewis base, the generic curved arrow formalism has even less meaning than normal, even though we can still use it to figure out what the product looks like. A better approach is to look at the HOMO and LUMO of the reactive species and see if the HOMO of one will overlap with the LUMO of the other in such a way that the phases match up on both sides. We saw that this does work for the Diels-Alder reaction but won't for other similar reactions (for instance the reaction between two equivalents of diene to make a cyclooctene). This makes it imperative to learn how to draw molecular orbitals for any of the relatively short-chain alkenes or polyenes (ethenes, dienes, trienes).

I have added a Molecular Orbital problem set on the left. I'll get a key ready for that (and for last year's exam) done ASAP.