Re: why is resolution of a mixture of enantiomers difficult?

Enantiomers have molecular structures which are mirror images of each other. 
Most simple molecules are their own enantiomers, and so the enantiomers are
impossible to separate! But even if a molecule has a mirror image which is
different from itself, enantiomers have identical physical properties.

Since most separations and purifications are done by physical processes, this
makes enantiomers very hard to separate. In fact, the only way to resolve 
a pair of enantiomers is to react them chemically with something else which
will convert them into compounds which no longer have mirror-image structures.
These compounds can then be separated, and to recover the enantiomers one 
then has to do chemistry to break the new compounds down into their original
ingredients.

Actually this is not quite true. Some ingenious and expensive work has been 
done to develop physical methods of separation which can distinguish between
enantiomers. But they work on the same principle given in the previous 
paragraph; it's just that the "new compounds" have such weak bonds that they
fall apart almost immediately. It's the "almost" that does the separation, and
the "immediately" that allows you to avoid doing any extra chemistry. (If
you know how chromatography works, the stationary phase is such that it has
a stronger affinity for one enantiomer of a pair.)

I hope that this answer is not too confusing; I didn't want to go into the
theory of chromatography and molecular recognition, too!

                                                   Dan Berger
                                                   MadSci Administrator