Find out the major product for the following reaction:

Step 1: Acid-catalysed dehydration of 1-methylcyclopentan-1-ol

1-Methylcyclopentan-1-ol is a tertiary alcohol (OH and CH3 both on C1 of the ring).

Protonation of -OH → oxonium ion → loss of water → tertiary cyclopentyl carbocation at C1.

Step 2: Ring expansion (1,2-alkyl shift)

The tertiary cyclopentyl carbocation can undergo a ring expansion via 1,2-carbon shift:

• One of the C-C bonds of the ring migrates to the adjacent carbocation center.
• This converts the 5-membered ring carbocation into a 6-membered ring carbocation (cyclohexyl carbocation).
• The cyclohexyl carbocation (secondary or tertiary) is formed — ring expansion is thermodynamically favorable (6-membered ring > 5-membered ring stability).

Step 3: Elimination

The cyclohexyl carbocation loses a proton to give cyclohexene (or methylcyclopentene depending on which proton is lost).

The major product via ring expansion is a cyclohexene derivative.

Key conceptual principle:

Students often miss ring expansion in carbocation intermediates. When a cyclopentane ring carries a carbocation, 1,2-shift of a ring C-C bond to an adjacent cationic C converts 5-membered → 6-membered ring (more stable).

Key Points to Remember:

• Tertiary carbocations from cyclopentane systems can undergo ring expansion via 1,2-C shift to give cyclohexyl cations.
• 6-membered ring is more stable than 5-membered → ring expansion is thermodynamically favored.
• Ring expansion is a key rearrangement to watch for in acid-catalyzed reactions of cyclic alcohols.