Stereochemistry is the field of study that is concerned with the structures of molecules in three dimensions. The three-dimensional structure of a molecule is important because often molecules will exist as isomers. An isomer is one of two or more molecules that have the same molecular formula, but different arrangements of atoms and bonds. Isomers can be subdivided into constitutional isomers and stereoisomers. A constitutional isomer is one of two or more molecules that have the same molecular formula but different structural arrangements of atoms and bonds. A stereoisomer (or spatial isomer) is one of two or more molecules that have the same molecular formula and bond structure, but different arrangements of the three-dimensional orientation of the atoms.Chirality is the configuration, or handedness, of an asymmetric chemical compound. An atom will be a chirality center if the atom, almost always a carbon atom, is bonded to four different atoms or groups. A chiral molecule is a molecule that does not have a plane of symmetry and whose isomers cannot be interconverted by rotation or reflection. An achiral molecule is a molecule that has a plane of symmetry and whose isomers can be interconverted by rotation or reflection.
Comparison of Constitutional Isomers and Stereoisomers
Subdivision of Isomers
Enantiomers have the same molecular formulas, they are mirror images of each other, and they have identical physical properties. Since their physical properties are identical, it is impossible to separate enantiomers by conventional methods. Fortunately, diastereomers have different physical properties and can be separated using conventional methods. Separation of enantiomers is more complex and involves nonconventional methods such as resolution. Resolution is the process for the separation of racemic compounds into their enantiomers.
One common method for resolving a racemic mixture or any solution of enantiomers involves converting the enantiomers to diastereomers. Alternatively, resolution of enantiomers is accomplished using chromatography (separation of mixture) with chiral media, a solvent or system that is chiral. A diastereomeric interaction occurs between the media and the enantiomers in the solution, and the distinct enantiomers will move through the chromatography apparatus at different speeds, separating themselves. Additionally, because the media is a solid, there is no need to perform another reaction to separate the enantiomer.