Category: Conference

  • 201 Web Site: Changes & Updates

    I’ve made a number of changes on the web site this weekend. Let me know if there’s something that you’d like to see.

    • Classes page – this replaces the old Syllabus page. Go here to find out what we are covering and when. Also go here to download learning activities, answers, and other in-class handouts. Warnings: 1. I’m still working on the schedule between class #14 (RF before Fall Break) and Thanksgiving. 2. I haven’t found all of the old links to Syllabus yet – if you see one, let me know what URL it appears on.
    • Conferences page – Go here to download conference problems and answers
    • Exams page – Go here to find exam policies, dates, old exams. The latter part of the exam schedule hasn’t been set – stay tuned.
  • C12 H25 Cl O2

    Problem 2B on yesterday’s conference assignment contained the wrong formula for the intermolecular substitution product. It should be C12 H25 Cl O2.

    Other comments: problem #3 was basically covered in lecture so please flip the page and try your hand on the NMR problem (#4).

  • HW #1, Conference assignments, Study guide #2

    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.
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  • Did You Lose Your Section?

    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. (more…)

  • Alkene pi orbital energies

    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.

    L27.111308 answer to conf 11 MO diagram.jpg

    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.