Question

Why can diastereomers be separated?

Answer 1

Because they are different chemically and in principle should have differential solubilities.

Explanation:

A mixture of diastereomers, say `"RR"` and `RS`, have in principle differential solubilities, and may potentially be separated by fractional crystallization.

I write `"in principle"` because there is no guarantee that a chemist will be able to separate the diastereomers, and separation by such means involves careful and laborious bench work. If you are good at such work a pharmaceutical firm will employ you; most modern pharmaceuticals are chiral and these days must go through rigorous testing and optical resolution.

Of course, chemical firms and their clients learned the hard way; thalidomide was once prescribed as a drug to alleviate morning sickness in pregnant women. One isomer worked, and I believe it is still prescribed today for those afflicted with leprosy. The other isomer led to congenital birth defects. Poor optical quality control in its manufacture led to many instances of congenital harm to unborn children before the drug was withdrawn.