Neopentyl bromide, $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{CH}{}_2-\mathrm{Br}$, is heated in ethanol with no added base. The principal substitution product is

Direct SN2 attack is blocked by the tert-butyl group, so the slow reaction proceeds by ionisation with simultaneous 1,2-methyl migration, giving the tertiary cation $(\mathrm{CH}{}_3){}_2\mathrm{C}{}^{+}-\mathrm{CH}{}_2\mathrm{CH}{}_3$. Trapping by ethanol gives $(\mathrm{CH}{}_3){}_2\mathrm{C}(\mathrm{OEt})\mathrm{CH}{}_2\mathrm{CH}{}_3$, i.e. 2-ethoxy-2-methylbutane, with rearranged skeleton.