Ease of reactivity is greatly influenced by the ease with which the chemical bond between the reactive atoms breaks. The important factor that decided this is its bond energy. For the case of halogen halides the more the bond energy they have the probability to break them reduces and makes the reaction slow in general. For reaction with propene, the intermediate formed is;
As in the second step there is breaking of bond due to induction of polarity on the carbocation means that the higher the bond energy of the halogen halide, the less likely it will break and react with the carbocation. So the increasing order of reactivity of halo-halide is same as their increasing bond energies.
Therefore, HI >HBr>HCl is the increasing order of reactivity.
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