Propene reacts with HBr in the absence of peroxide. The major product is

Without peroxide the reaction is ionic. H${}^{+}$ adds to the carbon bearing more hydrogens ($\mathrm{CH}{}_2$) to give the more stable secondary cation $\mathrm{CH}{}_3-\mathrm{CH}{}^{+}-\mathrm{CH}{}_3$, which is captured by $\mathrm{Br}{}^{-}$. The major product is 2-bromopropane (Markovnikov).