Main article: stereochemistry The different types of isomers. ...

Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. Properties In chemistry and physics, an atom (Greek άτομον meaning indivisible) is the smallest possible particle of a chemical element that retains its chemical properties. ... In general, a molecule is the smallest particle of a pure chemical substance that still retains its composition and chemical properties. ... In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. ...

The two main types of stereoisomerism are:

• cis-trans isomerism
• optical isomerism

Cis-2-butene Trans-2-butene In chemistry, geometric isomerism or cis-trans isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ... Optical isomerism is a form of isomerism (specifically stereoisomerism) where the two different isomers are the same in every way except being non-superposable mirror images of each other. ...

Cis-trans Isomerism

Main article: Cis-trans isomerism Cis-2-butene Trans-2-butene In chemistry, geometric isomerism or cis-trans isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ...

For example a geometrical isomerism occurs when a double bond is present, because the pi bond involved prevents that bond from being "twisted" the same way that a single bond can be. A good example is 1,2-dichloroethene: C₂H₂Cl₂. Consider the two examples below: Electron atomic and molecular orbitals, showing a Pi-bond at the bottom right of the picture In chemistry, pi bonds (Ï€ bonds) are bonds with a single nodal plane containing the line segment between the two atoms. ... 1,2-Dichloroethene, commonly called 1,2-dichloroethylene or 1,2-DCE, is an organochloride with the molecular formula C2H2Cl2. ...

The two molecules shown above are cis-1,2-dichloroethene and trans-1,2-dichloroethene. This is more specifically an example of cis-trans isomerism. These two molecules are geometrical isomers because the two carbon atoms cannot be rotated relative to each other, due to the rigidity caused by the pi bond between them. Therefore, they are not "superimposeable" - they are not identical, and cannot take each other's place. Geometrical isomers have different chemical and physical properties and can exhibit dramatically different biological activity. Image File history File links Dichloroethene. ... In chemistry, geometric isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ... Electron atomic and molecular orbitals, showing a Pi-bond at the bottom right of the picture In chemistry, pi bonds (Ï€ bonds) are bonds with a single nodal plane containing the line segment between the two atoms. ...

Optical Isomerism

Main article: optical isomerism Optical isomerism is a form of isomerism (specifically stereoisomerism) where the two different isomers are the same in every way except being non-superposable mirror images of each other. ...

Optical isomers are stereoisomers formed when asymmetric centers are present, for example, a carbon with 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, most biological compounds, such as amino acids, occur as single enantiomers. As a result, different enantiomers of a compound may have substantially different biological effects. When a molecule has more than one source of asymmetry, two optical isomers may be neither perfect reflections of each other nor superimposeable: some but not all stereocenters are inverted. These molecules are called they are diastereomers, not enantiomers. Diastereomers seldom have the same physical properties. Optical isomerism is a form of isomerism (specifically stereoisomerism) whereby the different 2 isomers are the same in every way except being non-superimposable mirror images* of each other. ... In chemistry, two stereoisomers are said to be enantiomers if they are mirror images of each other. ... In chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. ...

References

• Columbia Encyclopedia. "Stereoisomers" in Encyclopedia.com, n.l., 2005, Link, December 7^th 2005.

External links

• The IUPAC definition of "stereoisomerism"
• The IUPAC definition of "geometric isomerism"
• Chin. Chem. Soc., Vol. 46(3) 283 (1999) On the Assignment of Anomeric Configuration (pdf)