In the presence of organic peroxides, addition of HBr to propene gives predominantly:

Kharasch peroxide effect: HBr in the presence of organic peroxides (e.g. benzoyl peroxide) adds **anti-Markovnikov** via a free-radical mechanism. Br ends up on the **less** substituted carbon. Product: 1-bromopropane. Why: the radical chain involves a Br• radical adding to the alkene first. Br• prefers the less-substituted carbon, forming the more stable carbon radical. Then HBr donates H• to complete the addition. Notably, the peroxide effect is **specific to HBr**. HCl and HI don't show it.