Question

Select the correct statement about pi-bonds in valence bond theory.
Answer
a. A carbon-carbon double bond consists of two pi bonds.
b. A pi bond is stronger than a sigma bond.
c. A pi bond between two carbon atoms restricts rotation about the C

Answer 1

c. A pi bond between two carbon atoms restricts rotation about the C—C axis.

because

Double and triple bonds each have another type of bond different from the initial single bond present between the two bonded atoms. The sigma bond, the initial single bond, is formed through a head-on overlap of two atomic orbitals. Rotation about this bond does not change the overlap, and so rotation is allowed.

But double and triple bonds have pi bonds, formed by side-on overlap between the atomic orbitals (typically either p or d orbitals). These bonds surround the sigma bond and were rotation to occur, the orbitals involved with the pi bonds would cease to overlap and these bonds would be broken.

Therefore, the specific bond that restricts rotation about the axis of a double or triple bond would be the pi bond(s) involved.