Can optical isomers be separated?

1 Answer
Apr 15, 2016

In principle, they can be, but the chemical properties of each optical antipode are the same.


What do I mean by the above?

The chemistry of an enantiomer is identical to the chemistry of the other enantiomer, in all achiral environments. The key word is achiral.

Suppose you have a mixture of #R# and #S# isomers in roughly equal amounts. Further suppose that they are salts of some sort. Now you throw in an homochiral reagent, #R'#. What is this? It is a chemical from the so-called chiral pool, i.e. sugars, bases, acids, that are available from biological sources in quantity, and therefore have a specific handedness.

Let's further suppose that the added reagent #R'# reacts with your enantiomeric mixture. Now you have two species, #R'R# and #R'S#. These salts are still optically active BUT they are NOT mirror images. They are therefore diastereomers that may be separated by physical or sometimes chemical means.

Typically, the diastereomers would be separated by fractional crystallization. And this is the world of fractional crystaliization to the #n^(th)# degree. This is a tedious and time consuming process. It is sobering to think, however, that when we take drugs or medication, the drugs we routinely consume have gone through a resolution in which one optical isomer has been separated from its mirror image.

In recent years, chiral resolutions have become widespread. Here, instead of using crystallization, a chiral reagent is added that reacts preferentially with the one optical isomer (it can do so because this interaction is diasteromeric not enantiomeric). So, here one of the isomers is chemically transformed in a so-called chiral resolution. Because you chemically altered this isomer, it is a lot easier to separate from its original enantiomer.