Question

Assertion : Rotation about $C=C$ is restricted.
Reason : Electron charge cloud of the $\pi$ bond is located above and below the plane of bonding atoms.

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (B)

In a $\pi$ bond formation, parallel orientation of the two $p$ orbitals on adjacent atoms is necessary for proper sideways overlap. The $p$ orbitals are mutually parallel and both the $p$ orbitals are perpendicular to the plane of the molecule. Rotation of one $C{H}_2$ fragment with respect to other, interferes with maximum overlap of $p$ orbitals and hence such rotation is restricted.