In the addition of the hydrogen, halide to unsaturated alkenes in the presence of peroxides the halide adds to the carbon atom correlated to more number of hydrogen atoms and hydrogen adds to the carbon atom simultaneous to lesser number of hydrogen atoms. This is known as Peroxide effect (or) Kharasch effect.
CH3-CH2-CH2BRàCH3-CH=CH2+HBr-
N Prophyl bromide