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Cis–trans isomerism
Cis/trans isomerism (''geometric isomerism'', ''configurational isomerism'') is a term used in organic chemistry to refer to the stereoisomerism engendered in the ''relative'' orientation of functional groups within a molecule. It is not to be confused with ''E''/''Z'' isomerism, which is an ''absolute'' stereochemical description, and only to be used with alkenes. In general, such isomers contain double bonds that cannot rotate, or they may contain ring structures, where the rotation of bonds is restricted or eliminated.〔William Reusch, 2010, "Stereoisomers Part I," In ''Virtual Textbook of Organic Chemistry,'' Michigan State University, see (), accessed 7 April 2015.〕 Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes. Cis and trans descriptors are not used for cases of conformational isomerism where the two geometric forms easily interconvert, such as most open-chain single-bonded structures; instead, the terms “syn” and “anti” would be used.
The terms “cis” and “trans” are from Latin, in which ''cis'' means "on this side"〔(Charlton T. Lewis, Charles Short, A Latin Dictionary (Clarendon Press, 1879) ) Entry for ''cis''〕 and ''trans'' means "on the other side" or "across". The term "geometric isomerism" is considered an obsolete synonym of "cis/trans isomerism" by IUPAC.
== 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 but-2-ene.
Alicyclic compounds can also display cis/trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:
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