Enantiomers are allowed free rotations of the bonds, as they are characterized by single bonds. However, this freedom is taken away when a double bond or a cyclic structure is introduced in the molecule. In the case of alkenes, for example, all the atoms lie on the same plane and the structure is rigid, thanks to the double bond. The presence of a double bond gives importance to the position of the two groups connected to each carbon, whether they are above or below the double bond. This leads to a special kind of isomerism, part of the diastereomers, called cis/trans isomers. Having a double bond, however, doesn’t necessarily imply the presence of an isomer, as some conditions have to be fulfilled:

  • The rotation in the molecule must be restricted.
  • The molecule must present two nonidentical groups on each doubly bonded carbon atom.

If these conditions are fulfilled and the two groups of interest lie on the same side of the double bond, the isomer is classified as cis; if the two groups are on opposite sides, the isomer is called trans.

The cis isomer has both the chlorines connected to the two carbons lie on the same side of the double bonds, while the trans isomer has the chlorines on the opposite sides of the double bond.

In case three or four nonidentical groups are connected to the carbons forming the double bond, the cis/trans nomenclature is no more sufficient to describe the structure. It is required to apply the rules of prioritization of enantiomers to the two ligands on each carbon: if the groups with the highest priority on the two carbons lie on the same side, the isomer is described as Z; if they lie on opposite sides, it’s described as E.