In chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted.
The term "geometric isomerism" is considered an obsolete synonym of "cis-trans isomerism" by IUPAC. It is sometimes used as a synonym for general stereoisomerism (e.g., optical isomerism being called geometric isomerism); the correct term for non-optical stereoisomerism is diastereomerism.
There are two forms of a cis-trans isomer, the cis and trans versions. When the substituent groups are oriented in the same direction, the diastereomer is referred to as cis, whereas, when the substituents are oriented in opposing directions, the diastereomer is referred to as trans. An example of a small hydrocarbon displaying cis-trans isomerism is 2-butene.
Alicyclic compounds can also display cis-trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:
In the case of geometric isomers that are a consequence of double bonds, and, in particular, when both substituents are the same, some general trends usually hold. These trends can be attributed to the fact that the dipoles of the substituents in a cis isomer will add up to give an overall molecular dipole. In a trans isomer, the dipoles of the substituents will cancel out due to their being on opposite site of the molecule. Trans isomers also tend to have lower densities than their cis counterparts.
The cis/trans system for naming isomers is not effective when there are more than two different substituents on a double bond. The E/Z notation should 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 (higher atomic numbers are given higher priority). For each of the two atoms in the double bond, it is necessary to 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.
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