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STEREOISOMER
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Stereoisomers are isomeric molecules whose atomic connectivity is the same but whose atomic arrangement in space is different.
Enantiomers
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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.
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.
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(natural) tartaric acid
L-(+)-tartaric acid
dextrotartaric acid
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D-(-)-tartaric acid
levotartaric acid
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mesotartaric acid
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(1:1)
DL-tartaric acid
"racemic acid"
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Cis-trans isomerism
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This type of isomerism, also known as geometrical isomerism, can occur when a double (pi) bond or a ring is present. Bond rotation is restricted in these types of structures, so atoms can be permanently on the same (cis) or on opposite (trans) sides of them.
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 cis-1,2-dichloroethene (chlorines on same side - the top) and molecule II (chlorines on different sides) is trans-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 cis- and trans- 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.
Simple geometrical isomers usually have the same chemical properties, but different physical properties. More complex ones can exhibit dramatically different biological activity.
Conformers
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Conformational isomerism is a form of isomerism and describes the phenomenon of molecules with the same structural formula having different chemical conformations or conformers due to atoms rotating about a bond. Due to the low interconversion barriers between conformers, they are generally considered the same molecule. Therefore, one can say that for cyclo-hexane, the chair conformation is more stable than the boat conformation. There are some molecules that can be isolated in only one conformation, due to the energy difference between the several conformations.
References
- Columbia Encyclopedia. "Stereoisomers" in Encyclopedia.com, n.l., 2005, Link, December 7th 2005.
External links
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