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Knoevenagel Condensation: Mechanism & Examples

By Chemca Editorial Team • Last Updated: January 2026 • 9 min read

The Knoevenagel Condensation is a modification of the Aldol condensation. It involves the reaction of an aldehyde or ketone with a compound containing an Active Methylene Group in the presence of a weak organic base to form an $\alpha,\beta$-unsaturated compound.

1. What is an Active Methylene Group?

A methylene group ($-CH_2-$) is called "active" when it is flanked by two strong electron-withdrawing groups (EWG) like $-COOH, -COOR, -CN, -NO_2$, etc.

Examples: Malonic Acid, Ethyl Acetoacetate, Diethyl Malonate.

2. General Reaction

Condensation of Benzaldehyde with Malonic Acid in Pyridine yields Cinnamic Acid.

$$ C_6H_5CHO + \underbrace{CH_2(COOH)_2}_{\text{Malonic Acid}} \xrightarrow{\text{Pyridine}, \ \Delta} C_6H_5CH=CHCOOH + CO_2 + H_2O $$

Conditions:

• Substrate: Aldehyde or Ketone.
• Reagent: Active Methylene Compound ($Z-CH_2-Z'$).
• Catalyst: Weak Base (Pyridine, Piperidine, or Diethylamine).
• Product: $\alpha,\beta$-Unsaturated Acid (often followed by decarboxylation).

3. Detailed Mechanism

The mechanism involves nucleophilic addition followed by elimination and decarboxylation.

Step 1: Enolate Formation

The weak base (B:) abstracts an acidic proton from the active methylene group to form a resonance-stabilized enolate/carbanion.

$$ B: + H-CH(COOH)_2 \rightleftharpoons B^+H + \bar{C}H(COOH)_2 $$

Step 2: Nucleophilic Attack

The carbanion attacks the carbonyl carbon of the aldehyde.

$$ R-CHO + \bar{C}H(COOH)_2 \rightarrow R-CH(O^-)-CH(COOH)_2 $$

Step 3: Dehydration

Proton transfer and subsequent loss of a water molecule yield an unsaturated dicarboxylic acid.

$$ R-CH(OH)-CH(COOH)_2 \xrightarrow{-H_2O} R-CH=C(COOH)_2 $$

Step 4: Decarboxylation

Upon heating, compounds with two carboxyl groups on the same carbon (gem-dicarboxylic acids) lose $CO_2$.

$$ R-CH=C(COOH)_2 \xrightarrow{\Delta, \ -CO_2} R-CH=CH-COOH \text{ (Unsaturated Acid)} $$

4. Variations

Using Ethyl Cyanoacetate

Reaction with Ethyl cyanoacetate ($NC-CH_2-COOEt$) yields unsaturated cyano esters, which do not spontaneously decarboxylate.

$$ R-CHO + NC-CH_2-COOEt \rightarrow R-CH=C(CN)-COOEt $$

5. Perkin vs. Knoevenagel

Feature	Perkin Reaction	Knoevenagel Reaction
Nucleophile Source	Acid Anhydride	Active Methylene Compound
Base	Sodium Salt of Acid	Organic Base (Pyridine)
Product	Unsaturated Acid	Unsaturated Acid (via Decarboxylation)

Knoevenagel Quiz

Test your concepts on Active Methylene reactions. 10 MCQs with explanations.

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