Chem 201 Archive – Alan Shusterman

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

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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Halloween fell on Friday this year so I should have been expecting a “trick” or “treat” when I arrived at lecture. Little did I know that I would get both.

Everyone was seated in the dark when I arrived and two jack-o-lanterns were glowing on the front table. It took me a few moments to figure out that I should look at the front of them to see how they were carved. To my surprise, both had “organic” connections, glucose on the right (all equatorial chair conformer) and retinol (vitamin A) on the left.

Retinol is a rather long molecule so it required two photos from different angles to capture the entire molecule. Alex, thanks so much.

Left side of molecules (click for full size):

Right side of molecules (click for full size):

Here are pictures of the models that I shared with you in class on Monday. They address different issues.

Model #1 – A Stable Bromonium Ion. A bromonium can be stabilized by placing large groups around the alkene. These groups offer steric hindrance to the bromide anion so that backside attack can’t occur.

Notice that the “alkene” carbons in the reactant lose their planar geometry when bromine bonds to them. This geometry change pushes the bulky substituents downward where they block the path of any nucleophile that approaches from the backside. Since this also increases the exposure of the frontside of these carbon atoms, you might regard the stability of this ion as further evidence that SN2 reactions require backside attack.

View image

Model #2 – Unsymmetrical alkene leads to unsymmetrical bromonium ion, plus SN1-SN2 ring-opening. An unsymmetrical alkene like Me2C=CH2 produces a geometrically distorted bromonium ion like the following:

Notice that one CBr bond is much longer than the other, 2.39 v. 1.99 A. A normal CBr single bond is 1.95 A, so one bond in the bromonium ion is almost “normal” while the other is considerably weaker.

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I made two points about the 3-D aspects of nucleophilic substitution today in lecture:

1. the bimolecular reaction that I described (aka S_N2) occurs by “backside attack”. This is an experimental observation that has been made hundreds (probably tens of thousands) of times for a range of compounds. I could have added that molecular models also predict that backside attack is preferred, i.e., transition state models for backside attack are always lower in energy than transition state models for frontside attack;
2. the preference for backside attack can be rationalized by looking at how the nucleophile’s HOMO and the substrate’s LUMO overlap. Backside attack allows good overlap and the construction of a new, delocalized orbital in which the electrons originally residing on the nucleophile spread over the electrophilic carbon and the leaving group (see below left). The interactions in this three-atom orbital are bonding between Nu and C and antibonding between C and Lg. So, the nucleophile’s “attack” actually weakens the C-Lg bond.

It’s worth pointing out that the diagram in Solomons & Fryhle, p. 227 is labeled incorrectly. First, the phases or numerical signs of the orbital values are not shown, which is not helpful. Second, the “antibonding orbital” and “bonding orbital” labels make no sense; ignore them. On the other hand, you might find it helpful to re-read “The Chemistry Of … HOMOs and LUMOs in Reactions” on p. 97.