Rank the following in decreasing order of rate of solvolysis (SN1) in aqueous ethanol at 25 ${}^{\circ }$C: I $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{Br}$, II $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{Br}$, III $\mathrm{CH}{}_3\mathrm{CH}{}_2-\mathrm{Br}$, IV $(\mathrm{C}{}_6\mathrm{H}{}_5){}_3\mathrm{C}-\mathrm{Br}$.

SN1 rate follows carbocation stability. The trityl cation is delocalised over three phenyl rings and is the most stable, then the tertiary $(\mathrm{CH}{}_3){}_3\mathrm{C}{}^{+}$, then the secondary isopropyl cation, and the primary ethyl cation is the least stable. The order is IV > I > II > III.