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Compounds A and B undergo ozonolysis (O3 followed by Zn/H2O) to give the following products respectively. A gives succinaldehyde (butanedial) and glyoxal (ethanedial). B gives 5-oxohexanal. Identify A and B.

1. 1-methylcyclohex-1,3-diene and cyclopentene
2. cyclohex-1,3-diene and cyclopentene
3. 1-methylcyclohex-1,4-diene and 1-methylcyclopent-1-ene
4. cyclohex-1,3-diene and 1-methylcyclopent-1-ene

Correct answer: cyclohex-1,3-diene and 1-methylcyclopent-1-ene

Solution

Compound A: Cyclohex-1,3-diene on ozonolysis cleaves both double bonds (C1=C2 and C3=C4), producing succinaldehyde (OHC-CH2-CH2-CHO, from the C1-C2-C3-C4 fragment wait — actually C5-C6-C1 and C2-C3 fragments). The 4-C piece (C4-C5-C6-C1 with CHO at each end) is succinaldehyde, and the 2-C piece (C2-C3 with CHO at each end) is glyoxal. This matches. Compound B: 1-methylcyclopent-1-ene has C1(CH3)=C2 in a 5-membered ring. Ozonolysis opens the ring: C1 gives a ketone (CH3-CO-) and C2 gives an aldehyde (-CHO), yielding CH3-CO-CH2-CH2-CH2-CHO = 5-oxohexanal.

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