3-Methylbut-1-ene reacts with HBr in the dark. The major product is

Protonation gives the secondary cation $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{CH}{}^{+}-\mathrm{CH}{}_3$. A 1,2-hydride shift converts it into the more stable tertiary cation $(\mathrm{CH}{}_3){}_2\mathrm{C}{}^{+}-\mathrm{CH}{}_2\mathrm{CH}{}_3$, which is captured by bromide. The major product is 2-bromo-2-methylbutane, not the unrearranged 2-bromo-3-methylbutane.