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.

Spectroscopy -- Done!

Today was the end of NMR and we discussed multiplicity. We say the theoretical basis of the "N+1 Rule" and looked at a bunch of examples. The main point was essentially this: The integration is an indicator of how many hydrogens are responsible for the resonance and the multiplicity is an indication of the number of hydrogens on adjacent carbon atoms. We saw singlets, doublets, triplets and quartets. Complicated cases, where two or more different (non-equivalent) sets of hydrogens were nearby, led to multiplets.

We looked at the problem set and the 11 pages of the problem set key and saw the on-line problem set from Notre Dame (link at left). Each group, in lieu of a mash-up, is to give me the answers to the 12 listed problems by next Friday. Help each other!

1H NMR

Yesterday was a continuation of NMR, except we looked at the NMR of hydrogen rather than carbon. The concepts are the same, with an exception or two.....the main difference that we saw was that there is a quantitative aspect to 1H NMR that is not present in 13C NMR. It is possible to do an integration of the peaks, which will give us a relative ratio of the number of hydrogens responsible for each signal. Yesterday, all of the peaks were single peaks, on Friday we will look at how peaks can have different multiplicities -- there was some foreshadowing when I mentioned the N+1 Rule at the end of class.

Many changes have been made to the left: The Need to Knows for Chapters 13, 14 and 15 have been posted, as have the charts for 13C and 1H NMR. (Remember, you'll be given the 13C chart but must learn the 1H chart.) Modeling exercise #10 has also been posted; I'll talk more about it tomorrow.

New changes! A key for the exam and an NMR problem set have been posted. The problem set key will be posted once I get it cleaned up a bit and I'll have a group NMR activity for you tomorrow.

NMR

Today we started NMR spectroscopy. We covered the basics of 13C NMR and were exposed to some basics: upfield and downfield, shielded and deshielded, TMS, ppm, etc. The major point was that every unique carbon atom will have a resonance peak in a 13C NMR spectrum. This means that the most important thing that a student must worry about is determining how many unique carbon atoms are in a compound.

We also looked at two special 13C NMR experiments, DEPT-90 (only shows CH's) and DEPT-135 (shows CH and CH3 peaks pointing up, CH2 peaks pointing down).

IR Spectroscopy

Today we finished our short tour of MS and IR by looking at the basics of infrared spectroscopy. We reduced it down to a half-dozen or so types of bonds that will show up at different portions of the IR spectrum. It is anticipated that you will learn those areas and be able to draw IR spectra of simple molecules, including labels on the axes.

A key to last year's exam has been posted.

Spectroscopy

Today we started our treatment of spectroscopy by looking at mass spectroscopy (MS). MS is used by organic chemists primarily as a first look at a compound's molecular weight by observing the molecular ion peak. It is also valuable as a method for determining structure by looking at fragmentation patterns -- a molecule is ionized by bombarding it with an electron beam and the resultant radical cation then fragments into smaller particles. These particles can be detected provided they have a charge. We saw a limited number of fragmentations for which you will be responsible, including alpha cleavages, dehydrations and McLafferty rearrangements.

A new problem set and key has been posted. Last year's exam has also been posted.

The picture is of Asa Larson. He studies the process of an electron recombining with a molecular ion (the reverse of the step that starts MS) in his work at the Royal Institute of Chemistry in Stockholm, Sweden.