Question

Can pi bonds rotate?

Answer 1

No, pi bonds cannot rotate.

When a pi bond is formed, all the pieces of a molecule are locked in place.
This is one reason cis- and trans- isomers are formed.

Cis -atoms locked on same side after pi bond formation.
Trans -atoms locked on opposite sides after pi bond formation.

Notice how in Cisbutene, `H` and `CH_3` are on the same side as eachother. And in transbutene the `H` are opposite eachother, and `CH_3` are opposite eachother.

The biggest change between, cis and trans isomers is the polarity of the molecule.
Cis molecules tend to be polar whereas trans molecules tend to be nonpolar.