The bond strength order of halogen acid is `HF gt HCL gt HBr gt HI`. HF and HCl have high bond strength and polarity, so homolytic cleavage does not occur easily. In HBr and HI, homolytic clevage occurs readily to produce bromine and iondine free radicals. However, iodine radicals undergo coupling reaction with similar iodine radical to form iodiine `(I^(star)+I^(star) to I_(2))` molecule.
As a result radicals are not available for addition on alkenes. Thus, only HBr can provide free radical for addition to alkenes.
