Cis-Trans (Geometric) Isomerism
Requirements for Cis-Trans Isomerism
Cis-trans (geometric) isomerism occurs when two strict conditions are met:
- Restricted rotation about a double bond (\(\text{C=C}\)) or within a ring structure.
- Two different groups attached to each carbon of the double bond (e.g. \(a\text{C}=\text{C}b\) where each carbon has two distinct substituents).
Optical Isomerism (Enantiomers)
Requirements for Optical Isomerism (Enantiomers)
Optical isomerism occurs when a molecule contains an asymmetric (chiral) carbon atom: a carbon bonded to four different groups / atoms.
Enantiomers are non-superimposable mirror images that rotate the plane of plane-polarised light by equal angles in opposite directions.
Enantiomers Are Stereoisomers, Not Structural Isomers
Enantiomers have the exact same atom connectivity: they are stereoisomers, not structural isomers. Enantiomers share identical physical properties (same boiling point, melting point, density) EXCEPT their interaction with plane-polarised light and their behaviour in chiral environments (e.g. enzyme binding).
Identical Groups Prevent Cis-Trans Isomerism
A double bond with two identical groups on the same carbon (e.g. CH₂=CH-CH₃, prop-1-ene) CANNOT have cis-trans isomers because flipping the other end results in the exact same molecule.
Cis-Trans vs E/Z in IB Chemistry
The IB Chemistry syllabus specifically assesses cis-trans nomenclature. The CIP \(E/Z\) priority rules are not tested at Standard or Higher Level; focus on identifying cis (same side) vs trans (opposite sides) arrangements.
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