Geometric Isomerism

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 Bond s, 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 Diastereomer s.

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 Point s, lower Solubility in inert solvents, higher Heat Of Combustion (indicating higher Thermochemical stabilitiy) ''Advanced organic Chemistry, Reactions, mechanisms and structure'' 3ed. page 111 Jerry March ISBN 0471854727 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 Name s for 2-butenedioic acid and repectively the cis and trans isomer.

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

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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