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Benzaldehyde undergoes Cannizzaro reaction in concentrated NaOH solution to give two products: A (the alcohol) and B (the carboxylate). A is then oxidised by PCC to give aldehyde C. When C reacts with acetophenone (Ph-CO-CH3) in the presence of NaOH at elevated temperature, an aldol condensation-dehydration occurs, giving product D (the cross-aldol condensation product). Identify D.

1. Ph-CH=CH-CO-CH3
2. Ph-CH=CH-CO-Ph
3. Ph-C(CH3)=CH-CO-Ph
4. CH3-C(CH3)=CH-CO-Ph

Correct answer: Ph-CH=CH-CO-CH3

Solution

Cannizzaro reaction of PhCHO gives benzyl alcohol (A) and PhCOO- (B). PCC oxidises benzyl alcohol back to benzaldehyde (C). Aldol condensation of benzaldehyde (C) with acetophenone under NaOH/heat gives the alpha,beta-unsaturated ketone Ph-CH=CH-CO-CH3 after dehydration (D), which is benzalacetone (chalcone with methyl group).

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