The secondary bromide shown is warmed in aqueous ethanol containing silver nitrate. The cation undergoes successive 1,2-shifts to reach the most stable cation before capture by water. Which structure is the final substitution product?

The compound is (1-bromoethyl)cyclobutane. The secondary cation adjacent to the strained four-membered ring undergoes ring expansion (ring C-C bond migrates), giving the 2-methylcyclopentyl cation (secondary, strain relieved). A 1,2-hydride shift then gives the tertiary 1-methylcyclopentyl cation, captured by water to give 1-methylcyclopentan-1-ol.