How does a single covalent bond differ from a double covalent bond?
Single covalent bond involves both atoms sharing one atom which means there are two electrons in the bond. This allows the two groups on either side to rotate.
However, in a double covalent bond each atom shares two electrons which means there are 4 electrons in the bond. Since there are electrons bonded around the side, there is no way for either group to rotate, which is why we can have E-Z alkenes but not E-Z alkanes.