Why groups stay fixed around C=C
In stereoisomers, the atoms are joined together in the same sequence but occupy different arrangements in three-dimensional space. Suitable alkenes can therefore exist as E/Z stereoisomers.
Rotation can occur around a carbon-carbon single bond. In contrast, rotation about the planar C=C double bond is restricted.
The bond comes from sideways overlap between p orbitals on the two carbon atoms. Turning one carbon relative to the other would destroy this effective sideways overlap, so the substituents remain fixed relative to one another.
As a result, different arrangements around the double bond cannot simply interconvert by rotation.
Exam Tip: Explain the origin of E/Z isomerism by linking restricted rotation specifically to the bond in the C=C group.