Question

Why are there trans- and cis- isomers when it seems, at least to me, to make more sense to have it tilted about the bond axis 90 degrees more.

Answer 1

See what I mean? It keeps the atoms on either end farther apart, which would make more sense. I used to subscribe to an understanding that it was flat because the double bonds did it somehow, but now I don't really buy that anymore since I learned about resonance structures delocalizing electrons.

Answer 2

in order to have a double bond, the p-orbitals of the 2 atoms have to be perpendicular to the bond axis, but parallel relative to each other. The orbitals themselves overlap slightly, and they can't do that if they are both 90 degrees from each other as well.