2-Methylpropene, $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{CH}{}_2$, reacts with $\mathrm{Br}{}_2$ in water. The major product is

The bromonium ion is opened by water at the carbon better able to bear positive charge, the tertiary carbon. Br ends on the terminal $\mathrm{CH}{}_2$, and OH on the tertiary carbon. The product is $\mathrm{BrCH}{}_2-\mathrm{C}(\mathrm{OH})(\mathrm{CH}{}_3){}_2$, 1-bromo-2-methylpropan-2-ol.