Stereoisomerism

In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.

Enantiomers, also known as optical isomers, are two stereoisomers that are related to each other by a reflection: they are mirror images of each other that are non-superposable. Human hands are a macroscopic analog of this. 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 enantiomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects. Pure enantiomers also 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. Enantiomers differ by the direction they rotate polarized light: the amount of a chiral compound's optical rotation in the (+) direction is equal to the amount of its enantiomer's rotation in the (–) direction. ^{[citation needed]}

Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These include meso compounds, cis–trans isomers, 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.

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 familiar with only the latter naming convention. ^{[citation needed]}

A Fischer projection can be used to differentiate between L- and D-molecules (see Chirality (chemistry)). For instance, by definition, in a Fischer projection the penultimate carbon of D-sugars are depicted with hydrogen on the left and hydroxyl on the right. L-sugars will be shown with the hydrogen on the right and the hydroxyl on the left. ^{[citation needed]}

The other refers to optical rotation, when looking at the source of light, the rotation of the plane of polarization may be either to the right (dextrorotary — d-rotary, represented by (+), clockwise), or to the left (levorotary — l-rotary, represented by (−), counter-clockwise) depending on which stereoisomer is dominant. For instance, sucrose and camphor are d-rotary whereas cholesterol is l-rotary. ^{[citation needed]}

Stereoisomerism about double bonds arises because rotation about the double bond is restricted, keeping the substituents fixed relative to each other. If the two substituents on at least one end of a double bond are the same, then there is no stereoisomer and the double bond is not a stereocenter, e.g. propene, CH₃CH=CH₂ where the two substituents at one end are both H.

Traditionally, double bond stereochemistry was described as either cis (Latin, on this side) or trans (Latin, across), in reference to the relative position of substituents on either side of a double bond. A simple example of cis–trans isomerism is the 1,2-disubstituted ethenes, like the dichloroethene (C₂H₂Cl₂) isomers shown below.