Cis–trans isomerism

Cis–trans isomerism occurs when restricted rotation gives compounds different spatial arrangements of groups across a double bond or within a ring. These arrangements can produce distinct physical, chemical, or biological properties.

Stereoisomerism: Isomerism in which compounds have the same connectivity but differ in the spatial arrangement of their atoms. Cis–trans isomerism is one form of stereoisomerism.
Pi bond: A covalent bond formed by sideways overlap of parallel atomic orbitals. Breaking its orbital overlap is required for unrestricted rotation around an alkene double bond.
Cis–trans fat: An unsaturated fat whose fatty-acid chain contains a carbon–carbon double bond in cis or trans configuration. The double-bond geometry changes chain shape and influences fat properties.
Cis-2-butene: The 2-butene stereoisomer whose two methyl groups lie on the same side of the double bond. It is a simple example of the cis arrangement around an alkene.
Enantiomer: One of two stereoisomers that are nonsuperimposable mirror images of each other. Cis–trans isomers need not be mirror images and are often diastereomers.
Geometric isomerism: Stereoisomerism caused by restricted rotation that fixes groups in different relative positions. This is the traditional name for cis–trans isomerism.
Rotational barrier: The energy required to rotate one part of a molecule relative to another around a bond. A sufficiently high barrier keeps cis and trans configurations from rapidly interconverting.
Oleic acid: An 18-carbon monounsaturated fatty acid with a cis double bond at carbon 9. Its cis bond bends the chain, illustrating a common natural alkene geometry.
Trans-2-butene: The 2-butene stereoisomer whose two methyl groups lie on opposite sides of the double bond. Comparison with cis-2-butene makes the geometric difference explicit.
Diastereomer: A stereoisomer that is not the mirror image of another stereoisomer. Cis and trans forms of a molecule are commonly related as diastereomers.

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