Right vs. Left Handed Molecules

[Andromeda321]

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.

 

[chem_tr]

Hello, in organic chemistry, it is possible. We call it as S_N2 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 S_N2 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.

 

[movies]

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]

Chem tr, I can't seem to get that attachment to open.

 

[chem_tr]

Well, do you have winzip on your system, if not, get from here, or find a picture viewer capable of processing tif images.