Chem 201 Archive – Alan Shusterman

Category: Post-lecture

  • Internal Rotation & Hybrid Orbitals

    CH3 sp3 side and axial.jpg
    Today’s lecture on conformational isomers and internal rotation omitted a rather slick use of hybrid orbitals. Pictured above are two views of CH3 (methyl) with its singly occupied sp3 hybrid orbital on C (side view on left, orbital axis view on right). Notice the cylindrical symmetry of the orbital about the orbital axis.

    Now imagine CH3 were to bond to another CH3 to make ethane, C2H6. Both fragments would overlap their sp3 orbitals along their symmetry axes to make a bonding MO occupied by two electrons. Since both hybrid orbitals have cylindrical symmetry, they would overlap (and stabilize the electron pair) in exactly the same way for both a staggered and eclipsed conformer.

    This helps explain why conformers are so close in energy. Internal rotation can occur without weakening the bond that the groups rotate around.

  • Lecture slides – Wed Sept 8 (more MOs)

    Slides for today’s lecture are located here.

    Several of you came back with excellent questions after lecture today and I want to share some of this material with the rest of the class.
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  • 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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  • Lecture slides – Fri Sept 4

    Except for the computer troubles, I thought today’s lecture went surprisingly well. Too bad that we didn’t get through the entire set of slides, but I’ll show them to you on Monday. Here is a copy of the slides from today’s lecture.
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  • Lecture slides – Wed Sept 2

    You can download all of the slides (in color) from today’s lecture here.
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  • Lecture Moves to Psych 105

    It was incredibly crowded in Eliot 314 this morning. So, even though I love the room (it’s where I started teaching back in 1989), the sunlight, the traffic sounds from Eliot circle, the difficulty of seeing the white boards from certain locations, and I know you love it too …

    we will meet for lecture (a true *lecture*) in the Psych auditorium Wed, Sep 2, 9 AM. See you in our new location.

  • Cations, crowns, and phase transfer catalysts

    The following images (click image for full-size view) show potential maps (0 –> +1500), ball-and-spoke formulas, and space-filling formulas of

    • (CH3CH2CH2CH2)N+ (top left)
    • PhCH2N(Et)3+ (top center)
    • 18-crown-6(K+) (right)
    • Li+, Na+, K+ (bottom left)

    L35.120808 cations maps.jpgL35.120808 cations ballandspokes.jpgL35.120808 cations spacefillings.jpg
    The potentials surrounding the organic cations are more moderate, but not always for the same reason:

    • The positive charge in the quarternary ammonium ions is formally assigned to N, but N is buried under a blanket of C & H. The map surface lies rather far from N so the potential on the map surface is small.
    • The positive charge in the crown ether complex is assigned to K and this atom is exposed. However, the actual potential is the sum of extreme potentials generated by K and by the surrounding six O and these offset one another.

  • S/P electrophiles & crown ethers

    The PowerPoint slides from today’s lecture can be downloaded here. A couple of comments on each slide … (more…)

  • Huckel's rule, aromatic character, and molecular orbitals

    Today’s lecture tried to tie Huckel’s rule to a number of topics, including:

    • aromatic character, a more nuanced way of talking about aromaticity
    • charge distributions and geometry
    • evaluation of “minor” resonance structures
    • molecular orbital theory

    You can see all of the slides that were used here. I also remarked on the connection between various sources of polycyclic aromatic hydrocarbons (especially benzopyrene) and various sources of cancer. Here are some links that I found interesting:

    • benzopyrene – nice summary of chemical and biological phenomena including cancer discussion
    • In a Puff of Smoke – a site created by the Chemical Heritage Foundation for teachers (at what level? high school maybe?) that explores the science behind tobacco and other causes of cancer. Some facts that I discovered: U.S. per capita consumption of cigarettes rose from 94 per year in 1910 to 2,558 per year in 1940 (that’s per capita and since some people don’t smoke at all, consumption by smokers was actually much higher). Actually, there’s plenty of information here that I didn’t know.
    • Percivall Pott – the doctor who was the first to suggest a link between chimney soot and cancer

  • Models of Chemical Bonding

    Today I decided to re-trace my steps on Monday and provide a more complete description of chemical bonds through the eyes of molecular orbital theory. A list of take-home lessons from this presentation:

    • MO models contain new orbitals (molecular orbitals), while VB (valence bond) models do not
    • MOs can be mentally (de)constructed as combinations of atomic orbitals

    So far not much new stuff … the previous points had already been made in the context of the “localized MO” model presented at the start of the semester.
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