Which is the Knoevenagel condensation product between benzaldehyde and malononitrile?

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Multiple Choice

Which is the Knoevenagel condensation product between benzaldehyde and malononitrile?

Knoevenagel condensation forms an α,β-unsaturated product when an aldehyde or ketone reacts with an active methylene compound under base, followed by dehydration. Malononitrile has an especially acidic methylene between two strongly electron-withdrawing CN groups, so a base removes a proton to give a resonance-stabilized carbanion. That carbanion attacks the carbonyl carbon of benzaldehyde, forming a tetrahedral alkoxide intermediate. After proton transfers and loss of water, a carbon–carbon double bond is formed between the former carbonyl carbon and the methylene carbon, giving a conjugated product. The product is benzylidene malononitrile, with the structure Ph–CH=C(CN)₂, where the benzaldehyde part contributes the benzylidene moiety and the malononitrile part provides the two cyano groups on the vinylic carbon. This specific condensation product is the one formed in this reaction. Other options do not reflect this condensation pattern: they would arise from different reaction pathways (oxidation, self-condensation, or dimerization), not from the benzaldehyde–malononitrile Knoevenagel coupling.

Knoevenagel condensation forms an α,β-unsaturated product when an aldehyde or ketone reacts with an active methylene compound under base, followed by dehydration. Malononitrile has an especially acidic methylene between two strongly electron-withdrawing CN groups, so a base removes a proton to give a resonance-stabilized carbanion. That carbanion attacks the carbonyl carbon of benzaldehyde, forming a tetrahedral alkoxide intermediate. After proton transfers and loss of water, a carbon–carbon double bond is formed between the former carbonyl carbon and the methylene carbon, giving a conjugated product. The product is benzylidene malononitrile, with the structure Ph–CH=C(CN)₂, where the benzaldehyde part contributes the benzylidene moiety and the malononitrile part provides the two cyano groups on the vinylic carbon. This specific condensation product is the one formed in this reaction. Other options do not reflect this condensation pattern: they would arise from different reaction pathways (oxidation, self-condensation, or dimerization), not from the benzaldehyde–malononitrile Knoevenagel coupling.

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