Question

The reactivity of halogen acid with alkene to form haloalkanes are in the order:

• A. $HI>HBr>HCl>HF$
• B. $HF>HBr>HCl>HI$
• C. $HI>HF>HCl>HBr$
• D. $HI>HCl>HF>HBr$

Answer 1

The correct option is A $HI>HBr>HCl>HF$

Down the group in periodic table size of the halogen atom increases. Therefore, size of $HX$ ( X is halogens) increases in the order
$HF<HCl<HBr<HI$
As the size of molecule increase the bond length between H and X increases.
$\text{Bond length}\propto \frac{1}{\text{Strength of bond}}$
Therefore, bond strength increase in the order
$HI<HBr<HCl<HF$

Weaker the bond strength greater will be the reactivity HX toward addition.
Hence, the correct order of reactivity of halogen acid with alkene to form haloalkanes are is
$HI>HBr>HCl>HF$