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?

Explanation

Addition of halogen acids to an alkene is an electrophilic addition reaction.
`CH_(3)-CH=CH_(2)+H^(-)overset("slow")underset("1st step")rarrCH_(3)-overset(+)(CH)-CH_(3)overset(X^(-),"fast")underset("lind step")rarrCH_(3)-underset(X)underset(|)(CH)-CH_(3)`
First step is slow so, it is rate determining step. The rate of this step depends on the availability of proton . This in turn depends upon the bond dissociation enthalpy of the H-X molecule.
Lower the bond dissociation enthalpy of H-X molecule, greater the reactivity of halogen halide. Since the bond dissociation energy decreases in the order,
`HI(296.8 KJ mol^(-1)) lt HBr(363.7 KJ mol^(-1)) lt HCl(430.5 KJ mol^(-1))`
Therefore, the reactivity of the halogen acids decreases from Nl to HCl . i.e. , `Hl gt HBr gt Hcl`