Neopentyl bromide, (CH3)3CCH2Br, is heated in ethanol. The major substitution product is

The substrate is primary but hindered (neopentyl), so $\mathrm{S_N2}$ is extremely slow. Under solvolysis, ionisation is assisted by a 1,2-methyl shift, which converts the unstable primary cation into the tertiary $(\mathrm{CH_3})_2\mathrm{C^+CH_2CH_3}$. Capture by ethanol, then loss of $\mathrm{H^+}$, gives 2-ethoxy-2-methylbutane. The unrearranged ether needs a primary carbocation or $\mathrm{S_N2}$, and the alcohol would need water as the nucleophile.