Dehydration of menthol to alkene

Steps:

• Menthol, a secondary alcohol, undergoes acid-catalyzed dehydration with conc. H2SO4 upon heating.
• Protonation of OH group forms a good leaving group (water), generating a carbocation at C3.
• The carbocation loses a proton from adjacent carbon (C2 or C4) to form an alkene, following Zaitsev's rule for the most substituted product.
• Major product is 3-menthene, with double bond between C1-C2 in the cyclohexene ring.

Why D is correct:

• D shows the trisubstituted alkene (3-menthene), the thermodynamically stable major product per Zaitsev's rule.

Why the others are wrong:

• A: Exocyclic double bond, less stable and minor product.
• B: Isomer with double bond at C8-C9, not favored from menthol's carbocation.
• C: Fully saturated or incorrect substitution, violates dehydration mechanism.

Final answer: D