Question

which one of the following cannot disproportionate cl-, clo3-, cl2, cu+, fe2+

Answer 1

In order to disproportionate the ion must be in the "middle" of its range; i.e., Cl2 is at zero. It can go down (to -1) or up to +1 or higher. Fe^2+ can go down (to +1 or 0) or up (to +3). So the one you're looking for is either at the top or the bottom of its range.