Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but which differ only in the three-dimensional orientations of their atoms in space. Structural isomers share the same molecular formula, but the bond connections and/or their order between different atoms/groups differs. In stereoisomers, the order and bond connections of the constituent atoms remains the same, but their orientation in space differ.
Enantiomers are two stereoisomers that are related to each other by a reflection: they are mirror images of each other, which are non-superimposable. Human hands are a macroscopic example of stereoisomerism. Every stereogenic center in one has the opposite configuration in the other. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. For this reason, pure enantiomers exhibit the phenomenon of optical activity and can be separated only with the use of a chiral agent. In nature, only one enantiomer of most chiral biological compounds, such as amino acids (except glycine which is achiral), is present. As a result, different enantiomers of a compound may have substantially different biological effects.
Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These include meso compounds, cis-trans (E-Z) isomers, and non-enantiomeric optical isomers. Diastereomers seldom have the same physical properties. In the example shown below, the meso form of tartaric acid forms a diastereomeric pair with both levo and dextro tartaric acids, which form an enantiomeric pair.
(natural) tartaric acid
It should be carefully noted here that the D- and L-labeling of the isomers above is not the same as the d- and l-labeling more commonly seen, explaining why these may appear reversed to those only familiar with the latter naming convention. Please refer to Chirality for more information regarding the D- and L-labels.
Cis-trans and E-Z isomerism
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