3-Methylbut-1-ene is treated with $\mathrm{HBr}$ in the absence of peroxide. The major product is

Protonation gives the secondary cation $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{CH}{}^{+}-\mathrm{CH}{}_3$ (Markovnikov). A 1,2-hydride shift converts it to the more stable tertiary cation $(\mathrm{CH}{}_3){}_2\mathrm{C}{}^{+}-\mathrm{CH}{}_2\mathrm{CH}{}_3$, which captures $\mathrm{Br}{}^{-}$ to give 2-bromo-2-methylbutane as the major product. 2-Bromo-3-methylbutane is the minor unrearranged product.