Pairs of isomers can be enantiomers, diastereomers, or meso. If a compound has a plane of symmetry and a chirality center, it is a meso compound. Enantiomers are stereoisomers that are nonsuperposable mirror images of each other, and diastereomers are stereoisomers that are nonsuperposable and not mirror images of each other.
Enantiomers and diastereomers are stereoisomers that are nonsuperposable. Since superpose means to lay something upon another so as to make all like parts coincide, then nonsuperposable is an object if placed on top of another object they would not precisely overlap. This is more specific then superimposing objects, which is to place or lay over something. The only difference between them is that enantiomers are mirror images and diastereomers are not. Most diastereomers are either geometric isomers (cis/trans designation) or compounds containing multiple chiral centers. It is possible to predict the number of stereoisomers a molecule with multiple chiral centers will generate because the total number of stereoisomers will not exceed 2n, where n is equal to the number of chiral centers present in the molecule. However, a structure with two chiral centers does not always have four possible stereoisomers; sometimes, there are only three. Furthermore, molecules with multiple chiral centers may contain a plane of symmetry, which is an imaginary plane bisecting the molecule so that the two halves of the molecule are mirror images of each other. All molecules that contain a plane of symmetry are achiral. A meso compound is a molecule that is achiral even though it contains chirality centers. In other words, meso compounds contain a plane of symmetry and chiral centers. For example, in 2,3-difluorobutane, there are only three stereoisomers because two of the stereoisomers are identical. The stereoisomer with both fluorines dashed and the stereoisomer with both fluorines wedged are identical. If the stereoisomer with both wedged fluorine atoms were rotated 180°, it could be superposed onto the stereoisomer with both fluorines dashed. Additionally, the two stereoisomers are achiral because a plane of symmetry bisects the molecule between the carbon atoms with the fluorines. The other two stereoisomers (with one wedged fluorine and one dashed fluorine) do not have a plane of symmetry and are nonsuperposable mirror images or enantiomers.
Isomers can be categorized as constitutional isomers, stereoisomers, enantiomers, diastereomers, or even meso compounds. Knowing if a pair of stereoisomers are the same or what category to place them in is useful in categorizing molecules and determining reactions.
The steps for determining the relationship of two molecules are:
- If a pair of molecules have different molecular formulas, the molecules are not isomers and are unrelated.
- If a pair of molecules have the same molecular formula, the molecules are isomers.
- If the connectivity of the molecules is not the same, the molecules are constitutional isomers.
- If the connectivity of the molecules is the same, but they have a different orientation in space, they are stereoisomers.
- If the stereoisomers are nonsuperposable mirror images of each other, they are enantiomers.
- If the stereoisomers are not nonsuperposable mirror images of each other, they are diastereomers.