In organic chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism or E-Z 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. Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes.
The terms cis and trans are from Latin, in which cis means "on the same side" and trans means "on the other side" or "across". 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.
In organic chemistry
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:
Comparison of physical properties
Cis isomers and trans isomers often have different physical properties. Differences between isomers, in general, arise from the differences in the shape of the molecule or the overall dipole moment.
These differences can be very small, as in the case of the boiling point of straight-chain alkenes, such as 2-Pentene, which is 37°C in the cis isomer and 36°C in the trans isomer. The differences between cis and trans isomers can be larger if polar bonds are present, as in the 1,2-dichloroethenes. The cis isomer in this case has a boiling point of 60.3°C, while the trans isomer has a boiling point of 47.5°C. In the cis isomer the two polar C-Cl bond dipole moments combine to give an overall molecular dipole, so that there are intermolecular dipole-dipole forces (or Keesom forces) which add to the London dispersion forces and raise the boiling point. In the trans isomer on the other hand, this does not occur because the two C-Cl bond moments cancel and the molecule is non-polar.
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