In the following reaction, the left hand side and right hand side rings are named as A and B respectively. They undergo…

Step 1: Understand the starting material

The starting material is a bicyclic compound where ring A (4-membered cyclobutane) and ring B (5-membered cyclopentane) share a carbon bearing -OH at the ring junction.

Step 2: Acid-catalysed reaction — carbocation formation and ring expansion

When treated with acid ($\ce{H+}$, heat):

1. Protonation of -OH at the bridgehead → loss of water → carbocation at bridgehead.

2. The bridgehead carbocation undergoes ring expansion:

   • A C-C bond of ring A (cyclobutane, 4-membered) migrates → ring A expands from 4 to 5 members.
   • This creates a new carbocation in the ring system.

3. Second ring expansion:

   • Ring B (originally 5-membered) can also expand → from 5 to 6 members.
   • Ring A (now 5-membered from the first expansion) also becomes 6-membered after further migration.

Step 3: Final result

Both ring expansions proceed to give the thermodynamically stable product where both rings are 6-membered (cyclohexane fused ring system).

6-membered rings are the most stable (chair conformation of cyclohexane).

Answer: Finally both rings will become six membered each ✓

Key Points to Remember:

• Ring expansion in acid-catalyzed reactions proceeds toward thermodynamically more stable products.
• 6-membered rings are most stable; systems tend to expand toward this size.
• Each 1,2-shift expands a ring by one carbon (4→5, 5→6).
• Bridgehead carbocations facilitate ring expansion via bond migration.