When propene reacts with HBr, Markovnikov's rule predicts the major product is:

Markovnikov's rule: when HX adds to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogens, and X goes to the more substituted carbon. For propene $\mathrm{CH_3-CH=CH_2}$: H attaches to the terminal $\mathrm{CH_2}$, Br to the central $\mathrm{CH}$. Major product: $\mathrm{CH_3-CHBr-CH_3}$, which is 2-bromopropane. Mechanism: HBr protonates the alkene first, forming the more stable carbocation (secondary > primary). Bromide then attacks that carbocation.