1-Cyclobutylethyl bromide, $\mathrm{C}{}_4\mathrm{H}{}_7-\mathrm{CH}(\mathrm{CH}{}_3)-\mathrm{Br}$, is hydrolysed in water, and the alcohol A formed (via ring expansion and then a hydride shift to the most stable cation attainable) is treated with concentrated $\mathrm{HCl}$ and $\mathrm{ZnCl}{}_2$ at room temperature to give a chloride B. Compound B is

Ionisation gives the secondary cation $\mathrm{C}{}_4\mathrm{H}{}_7-\mathrm{CH}{}^{+}-\mathrm{CH}{}_3$. Migration of a ring bond to the cationic carbon expands the ring to the 2-methylcyclopentyl cation (secondary), and a hydride shift gives the tertiary 1-methylcyclopentyl cation; water gives A = 1-methylcyclopentan-1-ol. Treatment with $\mathrm{HCl}$/$\mathrm{ZnCl}{}_2$ protonates the OH, water leaves to give the same tertiary cation, and chloride attack gives B = 1-chloro-1-methylcyclopentane.