Explain the characteristics of $\pi$ bonds. Or,give the requirements for the formation of $\pi$ bonds and explain the active centers.

(N/A) In a $\pi$ bond formation,parallel orientation of the two $p$-orbitals on adjacent atoms is necessary for a proper sideways overlap.
Example: For the formation of a $\pi$ bond in $H_2C=CH_2$:
- All the atoms must be in the same plane.
- The $p$-orbitals are mutually parallel.
- Both $p$-orbitals are perpendicular to the plane of the molecule.
Effect of rotation on $C-C$ bond: In $CH_2=CH_2$,the rotation of one $CH_2$ fragment with respect to the other interferes with the maximum overlap of $p$-orbitals; therefore,such rotation about the carbon-carbon double bond $(C=C)$ is restricted.
Availability of electrons by $\pi$ bond and reactive centers: The electron charge cloud of the $\pi$ bond is located above and below the plane of the bonding atoms. This results in the electrons being easily available to attacking reagents. In general,$\pi$ bonds provide the most reactive centers in molecules containing multiple bonds.