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Geometric isomerism

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 strucutre, consider 1,2-dichlorocyclohexane:

trans-1,2-dichlorocyclohexane    cis-1,2-dichlorocyclohexane

Another notation can be used for cis and trans molecules. Z from the german Zusammen who means together can replace the term cis and E from the german Entgegen who means in opposite of can replace the term trans.

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