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Right vs. Left Handed Molecules

by Andromeda321
Tags: handed, molecules
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Andromeda321
#1
Feb5-05, 08:40 PM
P: 153
Yesterday in my chemistry class we were discussing right vs. left handed organic molecules and such and it got me wondering. First off, is there any way to make a right handed molecule left handed? Second, just how much would such a process cost? Thanks.
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chem_tr
#2
Feb6-05, 03:14 AM
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Hello, in organic chemistry, it is possible. We call it as SN2 reaction, in which an inversion (aka Walden's inversion) occurs, to render the molecule from, say, S to R.

I think you have not learned what S or R means; it is based on Cahn-Ingold-Prelog (CIP) nomenclature system.

Look at the attachment. Bromo is the functional group here; it can be substituted for hydroxide via an SN2 mechanism. With this way, pure enantiomers can be changed into desired one. It resembles upsiding-down of an umbrella when a strong wind blows.

About CIP system, you can do like this: Bromo is the largest atom in #1, so it is the first. Then comes the adjacent carbon with methyl group, since it contains a C-C bond instead of C-H in the others. The third one is the cyclohexyl CH2 group, where the last one in mass is the methyl. When you start with the first and go to the last one, you draw a clockwise curve, which means an R configuration. If it is anticlockwise, it is S, just in the product; hydroxide is #1, adjacent C with methyl is #2, adjacent CH2 is #3, and the methyl is #4. When you draw a curve (which is very hard to see indeed without molecular models), you'll see that it is anti-clockwise, hence S.
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#3
Feb6-05, 11:58 AM
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There are also processes called "dyanmic kinetic resolution" systems. These systems will typically take a racemic mixture (50/50 right- and left-handed molecules) and react them with a particular catalyst that reacts preferentially with one of the two (left or right) to make a different product (e.g., an alcohol oxidizing to a ketone). In that process you destroy the "handedness" of the molecule that reacted. In an dynamic kinetic resolution system there will typically be a second catalyst that will convert the product from the first reaction (e.g., the ketone) back into the starting material (the alcohol) with a preference for one particular handedness (left or right) of that product.

If you have a catalyst that selectively converts the left-handed alcohol to a ketone, and another catalyst to convert the ketone back to the right-handed alcohol then you can theoretically get 100% pure right-handed product.

Cool, huh?

Andromeda321
#4
Feb6-05, 04:08 PM
P: 153
Right vs. Left Handed Molecules

Chem tr, I can't seem to get that attachment to open.
chem_tr
#5
Feb6-05, 04:40 PM
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Well, do you have winzip on your system, if not, get from here, or find a picture viewer capable of processing tif images.


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