In Knoevenagel condensation, the newly formed double bond lies between which carbons of the carbonyl compound?

• A. Between the carbonyl carbon and the alpha carbon
• B. Between the beta carbon and the carbonyl oxygen
• C. Between the alpha and beta carbons
• D. Between the alpha carbon and oxygen

Answer: (C)

Knoevenagel condensation forms an α,β-unsaturated system, so the new C=C bond is placed between the α-carbon and the β-carbon relative to the carbonyl group. Mechanistically, the active methylene compound is deprotonated to a carbanion, which attacks the carbonyl carbon of the aldehyde or ketone. After dehydration, the resulting double bond links the carbon that was directly adjacent to the carbonyl (the α-carbon) with the carbon of the activated methylene fragment (the β-carbon). This creates the conjugated α,β-unsaturated framework characteristic of Knoevenagel products.