In the above reaction. Left hand side and right hand side rings are named as ' A ' and ' B ' respectively. They undergo ring expansion. The correct statement for this process is:
(A)Finally both rings will become six membered each.✓ Correct
(B)Finally both rings will become five membered each.
(C)Only 'A' will become 6 membered.
(D)Ring expansion can go upto seven membered rings
Answer: (A)
Solution
Protonation of the hydroxyl group and loss of water generate a carbocation on the carbon joining the two rings. Small rings carry large angle strain, so a ring C–C bond migrates to the electron-deficient centre, enlarging the ring and shifting the positive charge. The strained four-membered ring A expands first, and successive 1,2-shifts continue until each ring reaches the six-membered size, which has the least angle and torsional strain and gives the most stable carbocation. Hence both rings A and B finally become six-membered.
JEE Main · 2023 · 13 Apr 2023 · Shift 1 · Single correct option
