Cis-trans isomerism is a type of stereoisomerism in which compounds have the same structural formula and connectivity but different spatial arrangements. It occurs when restricted rotation prevents one arrangement from changing freely into the other.
The Reasoning and Mechanism
In an alkene, the carbon-carbon double bond contains one sigma bond and one pi bond. Rotation around the double bond would require the pi bond to break, so the groups attached to the double-bonded carbon atoms remain in fixed positions.
For an alkene to show cis-trans isomerism, each carbon atom in the double bond must be attached to two different groups. For example, but-2-ene qualifies because each double-bonded carbon is attached to one hydrogen atom and one methyl group.
| Isomer | Spatial arrangement |
|---|---|
| cis isomer | Identical groups are on the same side of the double bond |
| trans isomer | Identical groups are on opposite sides of the double bond |
Cis-but-2-ene and trans-but-2-ene are different compounds because they cannot interconvert without breaking the pi bond. They may have different physical properties, including boiling point and melting point, because their molecular shapes and overall polarities differ.
A common misconception is that every alkene displays cis-trans isomerism. Ethene and propene do not: at least one double-bonded carbon has two identical substituents. Cis-trans terminology also becomes inadequate when there is no identical pair of substituents across the double bond; the more general E/Z nomenclature is then used according to substituent priority.
Exam Technique
This is Additional Higher Level content under Structure 3.2. In an IB response, first identify the carbon-carbon double bond, then check both double-bonded carbon atoms for two different substituents. When drawing the isomers, keep the connectivity unchanged and show clearly whether the matching groups are on the same or opposite sides.
