🟣 This is Higher Level (HL) content.
Cis-Trans (Geometric) Isomerism
Requirement: Restricted rotation (C=C double bond) + two different groups on each carbon of the C=C.
- Cis. Same groups on the same side of the double bond
- Trans. Same groups on opposite sides
E.g. But-2-ene has cis and trans isomers (CH₃ groups same/opposite side).
Optical Isomerism (Enantiomers)
Requirement: A chiral centre. A carbon atom bonded to four different groups.
Enantiomers are non-superimposable mirror images. They have identical physical properties (same BP, solubility) but rotate the plane of polarised light in opposite directions.
⚠️ Examiner Trap
Enantiomers are not structural isomers. They have the same connectivity. They only differ in 3D spatial arrangement. If asked to draw an enantiomer, swap any two groups on the chiral centre.
⚠️ Cis-Trans Requires Different Groups
A C=C with two identical groups on one carbon (e.g. CH₂=CHCl) cannot exhibit cis-trans isomerism. Each carbon of the double bond must have two different substituents.
IB Note. E/Z Not Assessed
The IB specifically states that E/Z nomenclature will not be assessed. You only need to recognise and use cis and trans labels.