Dehydration of chiral secondary alcohol to single alkene

Steps:

• X is a pure optical isomer, so it has a chiral center and is likely a secondary alcohol.
• Heating with excess H2SO4 triggers acid-catalyzed dehydration via E1, forming a planar carbocation and then an alkene.
• For only one organic product, the reaction must yield a single alkene with no constitutional or geometric isomers possible.
• Option D's structure allows carbocation formation leading exclusively to a terminal alkene like styrene, which is achiral and unique.

Why D is correct:

• D is 1-phenylethanol; dehydration forms the benzylic carbocation PhCH(+)-CH3, which loses the methyl proton to give only PhCH=CH2 (Zaitsev's rule favors this conjugated alkene).

Why the others are wrong:

• A: Aliphatic secondary alcohol like 2-butanol yields mixture of 1-butene, cis-/trans-2-butene.
• B: Unsymmetric alcohol like 3-pentanol gives cis-/trans-2-pentene isomers.
• C: Primary alcohol like 2-methyl-1-propanol dehydrates slowly to isobutene but may form ethers or multiple products under excess acid.

Final answer: D