3,3-Dimethylbut-1-ene reacts with HCl at low temperature. The major product is

Protonation gives the secondary cation $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{CH}{}^{+}-\mathrm{CH}{}_3$. A 1,2-methyl shift gives the more stable tertiary cation $(\mathrm{CH}{}_3){}_2\mathrm{C}{}^{+}-\mathrm{CH}(\mathrm{CH}{}_3){}_2$, which is captured by chloride. The result is 2-chloro-2,3-dimethylbutane; the unrearranged 2-chloro-3,3-dimethylbutane is minor.