Organic Chemistry table of contents > Stereochemistry
Stereoisomers are compounds that have the same connectivity (constitution) and the same chemical formula, but are isomers because they differ in the spacial arrangement of the atoms attached to the stereocenters (chirality centers) throughout the molecule. All stereoisomers are unique and possess their own physical, chemical and biological properties (with the exception of meso compounds).
To illustrate the importance of stereoisomerism in organic chemistry, let's consider the well-known substance methamphetamine, which has two enantiomers: (R)-N-methyl-1-phenylpropan-2-amine and (S)-N-methyl-1-phenylpropan-2-amine. The S enantiomer, commonly called D-methamphetamine is a controlled substance in most countries worldwide. It is highly psychoactive, giving users an intense rush of euphoria when ingested. Use of D-methamphetamine often leads to severe drug dependence and even death in chronic users. By contrast, the R enantiomer of methamphetamine, commonly called L-methamphetamine is a non-psychoactive substance that is legal to possess in most jurisdictions. While not very psychoactive, L-methamphetamine is an excellent sympathomimetic vasoconstrictor and can be found in some OTC medications used to relieve nasal congestion caused by the common cold or allergic sinusitis, often under the name "levmetamfetamine".
Types of Stereoisomers
Enantiomers are chiral molecules with non-superimposable mirror images. When comparing stereoisomers, the enantiomer is always the isomer without any internal planes of symmetry and that has had each of its stereogenic centers flipped. In other words, each stereocenter has gone from an R-configuration to an S-configuration.
In this example, the two molecules are enantiomers of one another because (1) this molecule lacks any internal plane of symmetry and (2) all of its stereocenters have been flipped. The molecule on the right is (1S,3S)-3-chloro-1-ethylcyclohexane, while the molecule on the left is (1R,3R)-3-chloro-1-ethylcyclohexane.
Diastereomers are also chiral molecules, but differ from enantiomers in that they are not mirror images of each other due to some but not all of the chirality centers being flipped.
The following two molecules are diastereomers of one another because (1) they are not mirror images, and (2) only one of the stereocenters has been flipped. The molecule on the left is (1R,3S)-3-chloro-1-ethylcyclohaxane while the molecule on the right is (1S,3R)-3-chloro-1-ethylcyclohexane.
Meso compunds are achiral molecules that contain stereogenic centers. They are achiral molecules because their mirror images are superimposable onto each other as they possess an internal plane of symmetry, meaning that meso compounds are identical molecules with identical properties.
A common mistake is thinking that these two molecules are enantiomeric due to both stereogenic centers having been flipped. However, upon closer inspection these two molecules are not enantiomeric because by manipulating this molecule in space it could be superimposed on top of its mirror image due to it possessing an internal plane of symmetry.