Planning The Synthesis
Chirality must be introduced to the substance first. Then, it must be maintained. Care needs to be taken when planning the synthesis: a chemical change that makes the substance isotropic renders it achiral. This process is called epimerization. For example, a SN1 substitution reaction converts a molecule that is chiral by merit of non-planarity into a planar molecule, which has no handedness. (To visualise, draw the outlines of both of your hands on paper, and cut the images out. You can now superimpose the images, even if the hands themselves do not superimpose.) In a SN2 substitution reaction, on the other hand, the chirality inverts, i.e., when you start with a right-handed mixture, you'll end up with left-handed one. (A visualization could be inverting an umbrella. The mechanism looks just the same.)
Read more about this topic: Enantioselective Synthesis
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