Organic Chemistry Question

If I had to guess, I'd say that steric hindrance is the main reason there's no reaction with the cis form. With the chlorine sticking up on the same side as the hydroxyl group, the hydroxyl group (once it loses its proton with the addition of the base) can't reach over and attack the partially positive carbon atom that the chlorine is attached to. With the trans form, the chlorine atom is conveniently sticking down from the opposite face of the molecule, and handily sucking electrons away from the carbon.

Just a mildly educated guess. It's been a while since O-chem, and I only really took it b/c it was a prerequisite...