Geometric isomerism

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Image:Cis-2-butene.PNG Image:Trans-2-butene.PNG

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. Such bonds are typically double bonds, but they can also be part of a ring structure which prevents rotation.

According to IUPAC the term "geometric isomerism" is an obsolete synonym of "cis-trans isomerism" and its use is strongly discouraged. Sometimes the term "geometric isomerism" has been used as a synonym of stereoisomerism, i.e. optical isomers being considered to be geometric isomers. This, however, is not consistent with current standard chemical nomenclature. The exact term for stereoisomers that are not optical isomers is diastereomers.

There are two forms of a geometric isomer, the cis and trans versions. The form in which the substituent groups are on the same side of the bond that doesn't allow rotation is called cis; the form in which the substituents are on opposite sides of the bond is called trans. An example of a small hydrocarbon displaying cis-trans isomerism is 2-butene.

Cis isomers and trans isomers of a substance have different physical properties. Trans isomers generally have higher boiling points and higher densities. This is because the trans isomers molecules can line up and fit together better than the cis form. Two isomers with very different properties are maleic acid and fumaric acid. The names are two trivial names for 2-butenedioic acid and repectively the cis and trans isomer.

Alicyclic compounds can also display cis-trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:

Image:TrnC6H10Cl2.png	Image:CisC6H10Cl2.png
trans-1,2-dichlorocyclohexane	cis-1,2-dichlorocyclohexane

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E/Z notation

The trans/cis system for naming isomers breaks down when there are more than two different substituents on a double bond. The E/Z notation can then be used. Z (from the German zusammen) means together and corresponds to the term cis; E (from the German entgegen) means opposite and corresponds to the term trans.

Whether a molecular configuration is designated E or Z is determined by the Cahn Ingold Prelog priority rules. For each of the two atoms in the double bond, individually determine which of the two substituents is of a higher priority. If both of the substituents of higher priority are on the same side, the arrangement is Z; if they are on opposite sides the arrangement is E.

An example of a compound named in this manner is (Z)-1-bromo-1,2-dichloroethene.

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