Benzene is treated with 1-chloro-2-methylpropane ($(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{CH}{}_2\mathrm{Cl}$) and anhydrous $\mathrm{AlCl}{}_3$. The major product is

The primary isobutyl species rearranges by a 1,2-hydride shift to the tertiary tert-butyl cation. The tertiary cation is the most stable and is the species that alkylates benzene, so tert-butylbenzene is the major product. Isobutylbenzene could only be made reliably by acylation then reduction.