CH3CH=CH2+HBr peroxides CH3CH2CH2Br
Hydrogen Halides like HBr usually react with alkenes using an electrophilic addition mechanism. However, in the presence of organic peroxides, HBr adds to the carbon across the double bond which carries a lesser number of hydrogen atoms. This is called AntiMarkownikoff's rule.
This is called the peroxide effect or Anti-Markovnikovs rule.