A molecule (P) on treatment with acid undergoes rearrangement and gives (Q). (Q) on ozonolysis followed by reflux under alkaline condition gives (R). The structure of (R) is given below. [figure] The structure of (P) is
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Answer
(1) [figure], (4) [figure]
Solution
Working backwards from (R) (1-acetyl-2-methylcyclopentene):
(R) is the intramolecular aldol condensation product of octane-2,7-dione. In alkaline medium the enolate formed at C-3 attacks the C-7 carbonyl, giving a five-membered ring; dehydration then gives the conjugated enone (R):
CH3CO(CH2)4COCH3 [ ]OH- (R) + H2O
Octane-2,7-dione is the ozonolysis product of 1,2-dimethylcyclohexene, so (Q) = 1,2-dimethylcyclohexene ( C8H14).
(P) must give this alkene by a carbocation rearrangement in acid:
(1) Isopropenylcyclopentane: H+ adds to =CH2 3° carbocation C5H9- C(CH3)2 ring expansion (a ring bond migrates; strained five-membered ring becomes six-membered) cyclohexyl cation next to C(CH3)2 1,2-methyl shift gives a 3° cation loss of H+ gives 1,2-dimethylcyclohexene. Correct.
(2) 1-Isopropylcyclobutan-1-ol has only seven carbon atoms; it cannot give C8H14 (Q). Incorrect.
(3) 1,2-Dimethylcyclohexene is (Q) itself; it does not undergo any rearrangement with acid. Incorrect.
(4) 1-Isopropylcyclopentan-1-ol: protonation and loss of H2O 3° ring carbocation 1,2-hydride shift from the isopropyl CH gives the same 3° cation C5H9- C(CH3)2 as in (1) ring expansion, methyl shift and loss of H+ 1,2-dimethylcyclohexene. Correct.
Answer: (1), (4)