3-Methylbut-1-ene is treated with $\mathrm{Hg}(\mathrm{OAc}){}_2$ in water and then with $\mathrm{NaBH}{}_4$. The major product is

Oxymercuration proceeds through a bridged mercurinium ion, so no carbocation rearrangement occurs. Water attacks the more substituted carbon C-2, giving after demercuration 3-methylbutan-2-ol. The rearranged 2-methylbutan-2-ol is obtained only by acid-catalysed hydration.