JEE Main · 2023 · 11 Apr 2023 · Shift 1 · General Organic Chemistry

Protonation of the hydroxyl by HBr and loss of water first gives a secondary carbocation on the five-membered ring, adjacent to the angular carbon that bears the methyl group. This secondary cation rearranges by a 1,2-shift to the much more stable tertiary carbocation at the ring junction. Hyperconjugation then stabilises this tertiary centre: each ${\sigma }_{\mathrm{C}-\mathrm{H}}$ bond on the carbons $\alpha$ to the positive carbon (the methyl and the neighbouring ring methylene groups) overlaps with the empty p-orbital, and each such $\alpha$ C-H bond contributes one no-bond resonance structure. Counting all the $\alpha$ C-H bonds available to this carbocation, the number of hyperconjugative structures is 7.