# Why is the interhalogen I-Cl MORE reactive than Cl_2 or I_2 in electrophilic addition reactions?

Presumably because $I - C l$ has a definite dipole moment.......
The electronegativity of iodine is LESS than that of chlorine.......and thus we could represent the molecule as $\stackrel{\delta +}{I} - \stackrel{\delta -}{C} l$, i.e. with a permanent dipole. I have no data on the relative activities of ${I}_{2}$ versus $C {l}_{2}$ versus the interhalogen, as an electrophile, but we would predict that the reaction of $I - C l$ with $\text{propylene}$ would give $I - C {H}_{2} - C H C l - C {H}_{3}$ NOT $C l - C {H}_{2} - C H I - C {H}_{3}$.
And thus $I - C l$ is more polar than $C {l}_{2}$ or ${I}_{2}$............and thus expected to be more reactive. Anyway, if your text has data please consult them.