Question

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 $HIis430.5kJmo{l}^{-1}$,
$363.7kJmo{l}^{-1}$ and
$296.8kJmo{l}^{-1}$ respectively. What will be the order of reactivity of these halogen acids?

Answer 1

In this reaction, when propene reacts with HX, formation of intermediate carbocation takes place.

Now, more is the bond strength of $H-X$, more difficult it would be to break this bond and thus less will be the reactivity of these halogen acids with alkenes.

Given Bond energies of Halogen acids:

$HCl=430.5kJmo{l}^{-1}$, $HBr=363.7kJmo{l}^{-1}$, $HI=296.8kJmo{l}^{-1}$,

And
Bond energy $\alpha$ Bond strength of $H-X$

Thus, order of increasing bond strength of $H-X$ is:

$H-I<H-Br<H-Cl$

Hence, the order of reactivity of these halogen acids will be in reverse order as given below:

$H-I>H-Br>H-Cl$