Rotation of Pi-bonds

[LeAnn_Lee_1K]

Can someone explain why pi-bonds can not rotate around the bond axis? If this is true does that mean they are all on one plane? What is the significance of this?

[206363692]

pi-bonds are the side-to-side overlap of unhybridized p-orbitals, and the overlap forms an electron density region above and below the plane. To maintain the pi-bond, the p-orbitals have to remain parallel to each other. If one atom rotates relative to the other, the overlap between the p-orbitals is lost, breaking the pi -bond.