Stereoisomerism

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Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer.

The two main types of stereoisomerism are:

1 Cis-trans isomerism
2 Optical isomerism
3 References
4 External links

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Cis-trans isomerism

Main article: Cis-trans isomerism

These, also known as geometrical isomerisms occur when double bonds are present. Double bonds include a pi bond between atoms, around which no rotation can occur. An example is 1,2-dichloroethene, C₂H₂Cl₂. Consider the two molecules below: Image:Dichloroethene.png 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 swop 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.

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Optical isomerism

Main article: optical isomerism

Optical isomers are stereoisomers formed when asymmetric centers are present. This occurs when one or more atoms in the molecule, most often a carbon atom, has four different groups bonded to it.

Enantiomers are two optical isomers that are reflections of each other. Every stereocenter in one has the opposite configuration in the other. 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. 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 are molecules having more than one source of asymmetry. In this case, many optical isomers are possible, that may be neither perfect reflections of each other nor superimposeable, depending on which stereocenters are inverted. Diastereomers seldom have the same physical properties.

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