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An organic compound A is a hydrocarbon with molecular formula C6H10. Compound B, an isomer of A (also C6H10), gives CH3-CO-CH2-CH2-COOH as the sole product on oxidative ozonolysis. Compound A is converted to B via: (i) HOH, H+ (acid hydration), then (ii) conc. H2SO4, heat (dehydration). Identify the correct structure of A and B.

1. A is methylenecyclobutane (cyclobutane with exocyclic =CH2 and an extra -CH2- group); B is a 6-carbon ring-opened keto-acid
2. A is methylenecyclopentane (cyclopentane with exocyclic =CH2); B is obtained via ring expansion
3. A is ethylidenecyclobutane (cyclobutane ring with an exocyclic =CH-CH3 double bond); B is 1-methylcyclobutene
4. B is 1-methylcyclopentene (cyclopentene ring with a methyl group at C1)

Correct answer: A is ethylidenecyclobutane (cyclobutane ring with an exocyclic =CH-CH3 double bond); B is 1-methylcyclobutene

Solution

B is 1-methylcyclobutene (C5H8 — wait, C5H8 has MW=68). Actually ozonolysis product has 5 carbons: CH3(1C)-CO(1C)-CH2(1C)-CH2(1C)-COOH(1C) = 5 carbons, implying B is a 4-membered ring (4C ring + 1C substituent). B = 1-methylcyclobutene (C5H8, MW=68). But A and B are both C6H10, so B must be a C6H10 cyclopentene. Revising: CH3CO-CH2-CH2-COOH has formula C5H8O3 but counts 5 carbons — from a 5-membered cyclopentene ring. B = 1-methylcyclopent-1-ene gives ozonolysis product CH3CO-(CH2)3-CHO (6C) which oxidises to CH3CO-(CH2)3-COOH. The given product has only (CH2)2 chain between CO and COOH, implying B is a 4-membered ring cyclobutene derivative. This gives B as 1-methyl-but-1-ene ring = 1-methylcyclobutene with the ring having 4C + 1 CH3 = C5 total, MW=68 — not C6H10.

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