Day: November 5, 2008

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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