Dehydration of cyclohexanol followed by heating with acidified potassium permanganate gives:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Adipic acid
The conversion takes place in two steps.
First, cyclohexanol is dehydrated. An alcohol heated with a protic acid such as concentrated \( \ce{H2SO4} \) or \( \ce{H3PO4} \) loses a molecule of water and gives an alkene. Cyclohexanol gives cyclohexene.
Then the alkene undergoes oxidative cleavage. Hot acidified \( \ce{KMnO4} \) breaks a C=C bond completely. Each carbon of the double bond that carries a hydrogen becomes a –COOH group. In a ring, both new –COOH groups stay in the same molecule, so the ring opens into a dicarboxylic acid with all six carbon atoms:
\( \displaystyle \ce{C6H11OH} \xrightarrow[\Delta]{\text{conc. }\ce{H2SO4}} \underset{\text{cyclohexene}}{\ce{C6H10}} + \ce{H2O} \)
\( \displaystyle \ce{C6H10} + 4\,[\mathrm{O}] \xrightarrow[\Delta]{\ce{KMnO4/H^+}} \underset{\text{adipic acid}}{\ce{HOOC-(CH2)4-COOH}} \)
\( \ce{HOOC(CH2)4COOH} \) is hexanedioic acid, commonly called adipic acid.
Why the other options are incorrect
Option (1) Phthalic acid — Incorrect. Benzene-1,2-dicarboxylic acid is aromatic; it is obtained by oxidising o-xylene, not a saturated ring.
Option (3) Glutaric acid — Incorrect. \( \ce{HOOC(CH2)3COOH} \) has only five carbon atoms; it would come from cyclopentene.
Option (4) Malonic acid — Incorrect. \( \ce{HOOCCH2COOH} \) has only three carbon atoms.