JEE Main 2026 Apr 8 Shift 2, Chemistry Q13: Introduction to Mechanism of Organic Reaction

Test each option for whether the major product requires a carbocation rearrangement (hydride or methyl shift) or skeletal isomerisation.

Option (1): Friedel–Crafts alkylation of benzene with $n$-propyl chloride. The initially formed $1^{\circ }$ propyl cation rearranges via a hydride shift to the more stable $2^{\circ }$ isopropyl cation, so cumene (not $n$-propylbenzene) is the major product — rearrangement.

Option (2): Acid-catalysed dehydration of a $3^{\circ }$ alcohol. The protonation–loss of water gives a stable $3^{\circ }$ carbocation that directly loses a $\beta$-H to give the alkene — no skeletal rearrangement.

Option (3): AlCl${}_3$/HCl with $n$-hexane drives chain isomerisation to the branched 2-methylpentane — rearrangement.

Option (4): Acid dehydration where the initial cation undergoes a hydride/methyl shift before $\beta$-elimination, giving a rearranged alkene — rearrangement.

$\therefore$ The product of reaction (2) is obtained without rearrangement.