Question

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

Answer 1

Presumably because `I-Cl` has a definite dipole moment.......

Explanation:

The electronegativity of iodine is LESS than that of chlorine.......and thus we could represent the molecule as `stackrel(delta+)I-stackrel(delta-)Cl`, i.e. with a permanent dipole. I have no data on the relative activities of `I_2` versus `Cl_2` versus the interhalogen, as an electrophile, but we would predict that the reaction of `I-Cl` with `"propylene"` would give `I-CH_2-CHCl-CH_3` NOT `Cl-CH_2-CHI-CH_3`.

And thus `I-Cl` is more polar than `Cl_2` or `I_2`............and thus expected to be more reactive. Anyway, if your text has data please consult them.