Stereoisomerism

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Main article: stereochemistry

Stereoisomers are isomeric molecules whose atomic connectivity is the same but whose atomic arrangement in space is different.

1 Enantiomers
2 Diastereomers
3 Cis-trans and E-Z isomerism
4 Conformers
5 References
6 External links

[edit] Enantiomers

Main article: enantiomer

Enantiomers are two stereoisomers that are related to each other by a reflection: they are mirror images of each other. Every stereocenter 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 only be separated with the use of a chiral agent. In nature, only one enantiomer of most chiral biological compounds, such as amino acids, is present. As a result, different enantiomers of a compound may have substantially different biological effects.

[edit] Diastereomers

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 L-(+)-tartaric acid dextrotartaric acid	D-(-)-tartaric acid levotartaric acid	mesotartaric acid
(1:1) DL-tartaric acid "racemic acid"

[edit] Cis-trans and E-Z isomerism

Main article: Cis-trans isomerism

E-Z isomerism, is known as geometrical isomerism, and can occur when a double (pi) bond is present. Cis-trans isomerism occurs when a ring is present. In both of these types of structures, bond rotation is restricted, so atoms can be permanently on the same (Z- or cis) or on opposite (E- or trans) sides of a double bond or ring, respectively.

In a double (pi) bond, rotation of the bonded atoms is prohibited. An example is 1,2-dichloroethene, C₂H₂Cl₂. Consider the two molecules below:

Molecule I is Z-1,2-dichloroethene (chlorines on same side - the top) and molecule II (chlorines on different sides) is E-1,2-dichloroethene. As the two carbon atoms cannot be rotated relative to each other, there is no way of "superimposing" the structures on each other. Therefore, they are two different molecules.

In contrast, for 1,2-dichloroethane, C₂H₄Cl₂, which is similar except that it has an extra H attached to each C and a single bond, the E- and Z- forms do not exist. Since the carbon atoms can rotate around the single bond, in a flat projection of the molecule, all three atoms attached to one carbon could swap places and still represent the same structure.

Geometric isomers are diastereomers and can possess different physical, biological and chemical properties.

[edit] Conformers

Main article: Conformational isomerism

Conformational isomerism is a form of isomerism that describes the phenomenon of molecules with the same structural formula having different shapes due to rotations about one or more bonds. Different conformations can have different energies and can usually interconvert and are very rarely isolable. For example, cyclohexane can exist in a variety of different conformations including a chair conformation and a boat conformation, but these can never be separated, not least because a boat conformation represents a transition state on the conformational itinerary between the two equivalent chair forms. There are some molecules that can be isolated in only one conformation, due to the large energy barriers between different conformations.