##HCl## and ##HI## do not give anti-Markovnikov addition to alkenes because some of the steps in the chain reaction are endothermic.
Reaction with ##HBr##
The mechanism for anti-Markovnikov addition of ##HBr## is:
Step 1. Abstraction of ##H## from ##HBr## (highly exothermic).
##HO + H-Br HOH + Br##
Step 2. Addition of ##Br## to the alkene (exothermic)
##CH_3CH=CH_2 + Br CH_3stackrel()(C)H-CH_2Br##
Step 3. The carbon radical abstracts another ##H## from ##HBr## (exothermic)
##CH_3stackrel()(C)H-CH_2Br + H-Br CH_3CH_2-CH_2Br +Br##
The reaction is favourable because all the steps are exothermic.
Reaction with ##HCl##
The reaction with ##HCl## is unfavourable because the ##H-Cl## bond is much stronger than the ##H-Br## bond.
Step 1 is endothermic and the reaction becomes too slow to be useful.
Reaction with ##HI##
Step 1 is favourable because the ##H-I## bond is relatively weak.
But Step 2 addition of ##I## to the alkene is unfavourable and endothermic because of the bulk of the iodine radical.
Instead the iodine radicals combine to form ##I_2##.