Alkenes have a carbon-carbon double bond (>C = C<) in their molecules. The >C = C< bond is made up of one strong σ bond and one weak  bond. The electrons of the  bond are more exposed which increase reactivity. In other words, the presence of  bond (formed by the sideways overlapping of atomic orbital) makes alkenes highly reactive chemically as compared to alkanes. 

Diborane adds to alkenes to give trialkyl borane which on oxidation gives alcohol.

The >C = C< bond is made of σ bond and  bond. The  bond is weaker than σ  bond because electron in the  bond are more diffused in space i.e. orbitals participating in the  bond formation overlap sidewise only to small extent. On the other hand, orbitals participating in the σ bond formation overlap axially to a greater extent.




The greater the extent of overlapping, the higher the probability of finding the valence electrons in between the nuclei and hence the bond will be stronger and shorter. Hence overlap in  bond is less effective as compared to σ bond.

The mechanism of addition of HBr to a unsymmetrical alkene (say propene) in the presence of peroxide is free radical i.e. "H-Br undergoes homolysis to form free radical.



HCl is a very stable acid H-Cl bond (430 kJ moH) is stronger than H-Br bond (378 kJ mol^-1) and  is not broken symmetrically by the free radicals generated by peroxide. Hence the free radical addition of HCl to alkenes is not possible.
In the case of HI, the H - I bond (297 kJ mo^-1) is weaker than H-Br bond and undergoes homolysis readily to form iodine free radical. But iodine free radicals have greater tendency to combine amongst themselves to form iodine molecules rather than add to the ethylenic bond.



Hence HI does not respond to the peroxide effect.