Cis–trans isomerism
Geometric isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a molecule which has a double bond*. Sometimes geometric isomerism is used as a synonym of stereoisomerism, i.e. optical isomers are considered to be geometric isomers. The exact term for stereoisomers that are not optical isomers is diastereomers.
There are two forms, cis and trans versions of the isomer. The form in which the substituent groups are on the same side of the double bond is called cis, the form in which the substituents are on opposite sides of the double bond is called trans:
Cis and trans isomers of a substance have different physical properties. Trans isomers generally have higher boiling points and lower densities. This is because the trans isomer's molecules can line up and fit together better than the cis form.
- The molecule cannot rotate around a double bond, and so one isomer cannot turn into the other. Cyclic compounds also exhibit this form of isomerism and again, it is because the bonds cannot rotate - this time due to the ring structure. e.g. 1,2-dichlorocyclohexane:
trans-1,2-dichlorocyclohexane
cis-1,2-dichlorocyclohexane