Category: Post-lecture

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.

If you would like to look at the pictures of localized molecular orbitals (impress your parents! amaze your friends!) that I displayed in yesterday’s lecture, download the following PDF files.

Propene, CH3-CH=CH2

Formaldehyde, CH2=O

This question came up after lecture and its one worth bringing to the o chem public: Should sulfuric acid be drawn with double bonds (the way I originally drew it in lecture) or with single bonds+formal charges (the way I subsequently drew it)?

There is more here than meets the eye.
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