Friedel-Crafts

Today was the final day of new material and it ended with a bang -- the Friedel-Crafts alkylation and acylation reactions. These reactions involve the formation of C-C bond during EAS reactions. The carbon is made electrophilic by the reaction between an alkyl or acyl halide and a trihalo aluminum compound. The acylation is a better reaction (generally) than the alkylation because the resultant C+ is stabilized by resonance.

Lots of problem sets/keys/unkeyed exams are posted to the left.

More EAS

The topic for yesterday was how to handle an EAS reaction when there are multiple substituents on the ring. Essentially, it boiled down to this -- look at the directing effects of each substituent and determine the regiochemistry predicted by each. If they agree, life is good. If they disagree, then use the substituent that is more donating as the one that dominates the regiochemical outcome.

We also saw the synthesis of TNT. On paper, it's easy. In real life, it requires forcing conditions and nasty nitro group sources.

On to Greater Things....

Yesterday we started electrophilic aromatic substitution reactions, or EAS. The concept was generally this -- benzene is pretty unreactive as a Lewis Base, so in order to get it to react we need to increase the Lewis Acidity of the other reagent. This we did by using a separate Lewis Acid (with halides it was an iron halide) to put a positive charge on one of the halogen atoms (just like protonating -OH to make it water). We also saw that the carbocation intermediate, although losing stability due to loss of aromatic character, is somewhat stabilized by resonance. The reaction is finished off by deprotonation to regenerate the aromatic character of the ring.

A key for the exam has been posted to the left.

Exam #4!

In case you weren't paying attention, we had an exam today. I hope that it went well for you.

Keys added!

Keys for last year's exam and the Molecular Orbital problem set have been added to the left.

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.

Dienes

Yesterday was the first day to talk about 1,3-dienes. We covered two major topics -- first was a treatment of the molecular orbitals that come into play with dienes. There are four molecular orbitals (conservation of orbitals as we can think of them as being derived from the four p-orbitals of the pi-system) and the two lowest in energy each contain a pair of electrons. The two higher energy orbitals are not populated by electrons. We also identified the HOMO and LUMO, and discussed the concept of phases in orbitals. All of this was (hopefully) reinforced by showing the orbitals of cyclopentadiene, as determined by Spartan.

The second topic was the addition of HBr to a diene. We saw the possibility of two products and determined how to tweak the conditions (temperature) to allow us to get the kinetic or thermodynamic product.

The picture is of the four molecular orbitals of a diene, just as we discussed in class. It should expand if you click on it.