Some Stereochemistry Terms (Chirality); Structural Isomers, Stereoisomer, Enantiomer, Diastereomer, Meso

Some of the common terms used in chiral chromatography often need to be reviewed. Here are some of the most common ones we encounter in the chromatography lab each day. There are many detailed chemistry books on the topic, but I will try and present some simple explanations for each here.

"Structural Isomers": When two compounds have the same chemical formula, but different structures, then they are referred to as structural isomers. AKA "Constitutional isomers".

"Stereoisomers":  When two compounds have the same chemical formula and are connected to the same molecules, then they are referred to as stereoisomers.

"Enantiomers":  When two compounds have the same chemical formula and are connected to the same molecules (they are stereoisomers), but are also non-superimposable mirror images of each other, then they are referred to as chiral enantiomers. Enantiomers rotate the plane of plane polarized light to an equal extent, but in opposite directions.

"Diastereomer": When a stereoisomer compound has two or more chiral centers which are not mirror images of each other, then the compound is a diastereomer. IOW: Diastereomers are stereoisomers that are not enantiomers

"Meso Compounds": When a stereoisomer appears chiral due to the presence of more than two chiral centers (but is in fact not chiral), but is superimposable on its mirror image, then it is a meso compound. These are interesting forms and they are optically inactive because the two opposites cancel each other out (no +/- signal).