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Reference.org
Cis–trans isomerism
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<p><i>Cis</i>–<i>trans</i> isomerism, also known as geometric isomerism, describes certain arrangements of atoms within molecules. The prefixes "<i>cis</i>" and "<i>trans</i>" are from Latin: "this side of" and "the other side of", respectively. In the context of chemistry, <i>cis</i> indicates that the <a href="/facts/Functional_groups/GXkSPsjM">functional groups</a> (substituents) are on the same side of some plane, while <i>trans</i> conveys that they are on opposing (transverse) sides. <i>Cis</i>–<i>trans</i> isomers are <a href="/facts/Stereoisomers/I3kzlDE8">stereoisomers</a>, that is, pairs of molecules which have the same formula but whose functional groups are in different orientations in three-dimensional space. <i>Cis</i> and <i>trans</i> <a href="/facts/Isomers/PXlvDVdg">isomers</a> occur both in organic molecules and in inorganic coordination complexes. <i>Cis</i> and <i>trans</i> <a href="/facts/Descriptor_(Chemistry)/cPz4wDpX">descriptors</a> are not used for cases of <a href="/facts/Conformational_isomerism/JflrT5Ws">conformational isomerism</a> where the two geometric forms easily interconvert, such as most open-chain single-bonded structures; instead, the terms "<i>syn</i>" and "<i>anti</i>" are used. </p><p>According to <a href="/facts/IUPAC/2AV6myCK">IUPAC</a>, "geometric isomerism" is an obsolete synonym of "<i>cis</i>–<i>trans</i> isomerism". </p><p><i>Cis–trans</i> or geometric isomerism is classified as one type of configurational isomerism. </p>