In chemistry, geometric 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.
Sometimes the term "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 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.
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 isomers molecules can line up and fit together better than the cis form.
As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:
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| trans-1,2-dichlorocyclohexane | cis-1,2-dichlorocyclohexane
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Another notation can be used for cis and trans molecules.
Z from the German Zusammen, which means together can replace the term cis; and E from the German Entgegen who means in opposite of can replace the term trans. This alternate system of designation is necessary when there are more than two different substituents on a double bond. Whether a molecular configuration is Z or E is determined by the Cahn Ingold Prelog priority rules of the substituents. 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.