2,2-Dimethylpropan-1-ol (neopentyl alcohol) is heated with conc. $\mathrm{HBr}$. The main product is

Neopentyl alcohol cannot undergo a normal ${\mathrm{S}}_{\mathrm{N}}2$ because of steric hindrance at the neopentyl carbon. Ionisation is accompanied by a methyl shift (Wagner-Meerwein) to give the tertiary cation $(\mathrm{CH}{}_3){}_2\mathrm{C}{}^{+}\mathrm{CH}{}_2\mathrm{CH}{}_3$, which is captured by bromide to give 2-bromo-2-methylbutane.