Recently in Conference Category

I've been inundated with questions and comments about homework assignment #1, and requests for changes in conference assignments, so I wanted to bring you up to date on these and other items of interest.

The Registrar's Office has reassigned lab and/or conference sections for several students. Please check your class schedule this weekend to see whether it has been affected. If changes were made, they were made for a reason. You must attend your assigned section for now (note: labs do not meet this week), but there is a small chance (described below) that you can change your section again.

Many of you have gone out of town for the weekend and will return wondering what's going on in Chem 201. Here is some information about what's happening during this long weekend and the following week...

I'm catching up this weekend, but I won't get all caught up. Look for the following items to get posted during the next 48 hours:

• Chapter 13 study guide
• Homework X3 (this will cover chapters 13-14, it won't be collected, but answers will be posted)
• Maybe an old exam for review?

I'm also going to spend time grading lab reports, but the posted materials seem more important (and there are 110 reports to grade) so I might have to let the lab reports sit.

Next week will bring all kinds of new things:

• Lecture. Chapter 11 will be discussed in lecture on Monday and Friday
• Exam. Exam #3 will be on Wednesday. The Q&A session for exam #3 will be on Tuesday evening (location to be determined)
• Lab. We are formally finished with lab work, but you will need to come to the computer room this week to print your acetylferrocene NMR spectrum. Details on what to print and how to print it will be issued later. If you set your acetylferrocene sample in your desk so that it could dry, then you can measure its melting temperature in lab. Right now it looks like there is a conflict between our use of the computer room on Wednesday and Chem 101 so Wed folks should expect some delays. Lab reports will be due the following week.
  
• Conferences for next Thursday (Dec 4) are canceled. Conference sections will meet, however, the following week on Tuesday, Dec 9 (normal times & locations, just a strange day; important note - all Tuesday classes at the College are canceled on Dec 9 and Thursday classes meet instead - see Reed College academic calendar).
  

A question on today's conference problem set asked you to predict how the energies of the pi and pi* orbitals of a typical alkene would respond to twisting the alkene.

Valence bond fans: The VB model doesn't contain "pi" or "pi*" orbitals, but it correctly predicts that twisting the alkene destroys p-p overlap, i.e., destroys the pi bond and destabilizes the molecule.

The following picture shows the answer to the conference problem (click for larger image). The planar alkene is shown on the left and the fully twisted alkene on the right. The pi MO (bottom) and pi* MO (top) energies of the planar alkene are very different (vertical axis is MO energy). As we twist the alkene, the two MO energies converge.

To make this prediction you need to visualize each MO as being constructed from two 2p orbitals. If the overlap between these 2p "pieces" is "bonding" (no node + orbitals close together), as in the pi MO, it stabilizes the MO. Twisting reduces the overlap and destabilizes the pi MO. When the overlap disappears completely, the pi MO energy becomes equal to that of a 2p orbital.

The opposite behavior is seen when overlap between the 2p orbitals is "ANTIbonding" (node + orbitals close together). This kind of overlap destabilizes the MO and that's why the pi* MO is higher in energy. Twisting the alkene stabilizes the pi* MO because the antibonding overlap is reduced. When the antibonding overlap disappears completely, the pi* MO energy equals that of the pi MO.

Please bring a model kit to conference tomorrow. It will prove its value.

Please obtain a plastic model kit from the Chemistry stockroom (or a friend or whatever). They cost only $15 and if you use it (either on an exam or a homework assignment or just to clarify your thinking), you'll be glad you did.

Please bring your model kit to conference tomorrow. This will be our only chance prior to the exam to figure out together how to use the bloody thing.

Problem #1 was worded incorrectly. The problem sheet handed out stated, "no stereoisomerism is possible for the top compound, but it is possible for the bottom one." The compounds should have been reversed. Stereoisomerism is possible for the top, but not for the bottom.

A closer look at the IR spectrum is revealing. The arrows point to the bands created by C=C stretching vibrations in the two alkenes.