What is a characteristic of a pi bond in alkenes?

A characteristic of a pi bond in alkenes is that it prevents rotation around the double bond. In alkenes, a double bond is composed of one sigma bond and one pi bond. The sigma bond is formed by the head-on overlap of orbitals, allowing for free rotation. However, the pi bond is formed by the side-to-side overlap of p orbitals, which creates a region of electron density above and below the plane of the sigma bond. This arrangement restricts the rotation around the double bond because any rotation would disrupt the parallel alignment of the p orbitals required for the pi bond to remain intact.

This restriction on rotation is significant in organic chemistry, particularly in the stereochemistry of alkenes, as it can lead to different geometrical isomers (cis and trans forms) due to the fixed orientation of substituents attached to the double bond. Therefore, the ability of a pi bond to prevent rotation around the double bond is a key characteristic that distinguishes it from a sigma bond.