The compound that CANNOT be obtained from the aldol condensation reaction shown below, is [figure]
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Answer
(2) [figure]
Solution
In aldol condensation under basic conditions, elimination of water occurs via the E1cb pathway through an enolate intermediate adjacent to the carbonyl group.
Cross-aldol reaction between 2,2-dimethylcyclopentanone and benzaldehyde gives, after dehydration, the conjugated benzylidene enone, obtained as its two geometrical isomers (1) and (3).
Self-aldol condensation followed by dehydration towards the -position gives conjugated enone (4).
Compound (2) requires dehydration into the non-enolisable, unconjugated ring position, which cannot proceed via E1cb and lacks conjugation stability. Hence, (2) cannot be obtained.
Answer: (2)