Ncert Exercises & Solutions

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 willl be the order of reactivity of these halogen acids?

Addition of halogen acids to an alkene is an electrophilic addition reaction.

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}) < HBr (363.7 kJ {mol}^{- 1}) < HCl (430.5 kJ {mol}^{- 1})

Therefore, the reactivity of the halogen acids decreases from HI to HCl, i.e., HI > HBr > HCl

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