The intermediate carbocation formed in the reactions of HI, HBr and HCl with propene is the same and the bond energy of HCl, HBr and HI is 430.5 kJ mol-1, 363.7 kJ mol-1 and 296.8 kJ mol-1 respectively. What will be the order of reactivity of these halogen acids?
Reactivity of hydrogen halides depends on the dissociation enthalpy of H-X. The order of reactivity of hydrogen halides is HI > HBr > HCl. They add up to alkanes to form alkyl halides.
Bond enthalpy of HI<HBr<HCl, hence reactivity in reverse order is:
HCl < HBr < HI