Isomers - Geometric
- One of the features of alkenes is their stereoisomerism or geometric isomerism, which can be classed as E / Z or cis / trans
- E / Z isomerism is an example of stereoisomerism where different atoms or groups of atoms are attached to each carbon atom of the C=C bond
- Cis / trans isomerism is a special case of E / Z isomerism where two of the atoms or groups of atoms attached to each carbon atom of the C=C bond are the same
Cis / trans isomers
- In saturated compounds, the atoms / functional groups attached to the single, σ-bonded carbons are not fixed in their position due to the free rotation about the C-C σ-bond
- In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position
- This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond
- Cis / trans nomenclature can be used to distinguish between the isomers
- Cis isomers have two functional groups on the same side of the double bond / carbon ring, i.e. both above the C=C bond or both below the C=C bond
- Trans isomers have two functional groups on opposite sides of the double bond / carbon ring, i.e. one above and one below the C=C bond
The presence of a π bond in unsaturated compounds restricts rotation about the C=C bond forcing the groups to remain fixed in their position and giving rise to the formation of certain configurational isomers