Why cis-trans stereoisomers exist
Why is there "restricted rotation" about a double bond?
A double bond is, as its name implies, two bonds. One of these bonds is the same as the other single bonds in the molecule.
The two electron density pictures below show separately the electron density map for the single bonds in ethene (four C-H and one C-C) and the electron density map for the "second bond" of the C=C in ethene.
For singly-bonded atoms, the electron density is highest around the bond axis (line connecting the bonded atoms). The two atoms joined by a single bond can rotate with respect to one another without affecting the overlap.
Doubly bonded atoms are joined by a single bond and a second bond known as a pi (π) bond. π Bonds are formed by overlap of two p-orbitals (dumbbell-shaped) on adjacent carbons.
The electron density for the π bond is above and below the bond axis (line joining the two carbons). If one carbon and its hydrogens in ethene is rotated with respect to the other one, the overlap (bond) is destroyed. Thus rotation about a carbon-carbon double bond is restricted.
A double bond is, as its name implies, two bonds. One of these bonds is the same as the other single bonds in the molecule.
The two electron density pictures below show separately the electron density map for the single bonds in ethene (four C-H and one C-C) and the electron density map for the "second bond" of the C=C in ethene.
For singly-bonded atoms, the electron density is highest around the bond axis (line connecting the bonded atoms). The two atoms joined by a single bond can rotate with respect to one another without affecting the overlap.
The electron density for the π bond is above and below the bond axis (line joining the two carbons). If one carbon and its hydrogens in ethene is rotated with respect to the other one, the overlap (bond) is destroyed. Thus rotation about a carbon-carbon double bond is restricted.