Knoevenagel condensation template

Piperidine deprotonates malononitrile; the anion adds to benzaldehyde and E1cB loss of hydroxide gives benzylidenemalononitrile.

Mechanism step by step

An active methylene compound condenses with an aldehyde under mild base

1. Carbanion – Two C≡N groups make the CH₂ acidic (pKa 11)
2. Addition – Attack on the aldehyde
3. Protonation – The piperidinium ion gives back its proton
4. Carbanion – The remaining acidic C–H is removed
5. Elimination (E1cB) – Hydroxide leaves; C=C forms in conjugation

Key points

• Needs two electron-withdrawing groups on the CH₂ (malononitrile, malonates, cyanoacetates, β-keto esters).
• A weak amine base is enough, so the aldehyde does not self-condense.
• Doebner modification: malonic acid in pyridine also decarboxylates, giving cinnamic acid.
• Piperidine can also act through an iminium ion of the aldehyde.

Drawn with 179 atoms, 16 arrows, 7 shapes – every one editable in MolDraw.

Sources

• Knoevenagel, Ber. Dtsch. Chem. Ges. 1898, 31, 2596 Original paper
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook