Claisen & Claisen-Schmidt Condensations
While Aldol condensation involves aldehydes and ketones, the Claisen Condensation deals with Esters. It is a fundamental C-C bond-forming reaction used to prepare $\beta$-Keto Esters. A related variation, the Claisen-Schmidt Condensation, is a cross-aldol reaction between an aromatic aldehyde and an aliphatic ketone.
1. Claisen Condensation
It involves the self-condensation of two molecules of an ester containing $\alpha$-hydrogen atoms in the presence of a strong base (like Sodium Alkoxide) to form a $\beta$-Keto Ester.
Conditions:
- Substrate: Ester with at least two $\alpha$-Hydrogens.
- Reagent: Strong base matching the ester's alcohol group (e.g., $NaOEt$ for Ethyl esters) to avoid transesterification.
- Product: $\beta$-Keto Ester.
Mechanism of Claisen Condensation
This reaction follows a Nucleophilic Acyl Substitution pathway.
- Enolate Formation: Base abstracts an $\alpha$-proton from the ester to form an enolate ion.
- Nucleophilic Attack: The enolate attacks the carbonyl carbon of a second ester molecule.
- Elimination: The ethoxide ion ($EtO^-$) leaves, reforming the carbonyl double bond.
- Deprotonation (Driving Force): The formed $\beta$-keto ester is quite acidic (active methylene). The base removes a proton from it, forming a stable enolate salt. This step drives the equilibrium forward.
2. Claisen-Schmidt Condensation
This is a specific type of Cross-Aldol Condensation between an aromatic aldehyde (no $\alpha$-H) and an aliphatic aldehyde or ketone (with $\alpha$-H) in the presence of a dilute base ($NaOH$).
Key Features:
- Reactant 1: Aromatic Aldehyde (e.g., Benzaldehyde) - Acts as the Electrophile (Acceptor).
- Reactant 2: Aliphatic Ketone/Aldehyde (e.g., Acetophenone, Acetone) - Acts as the Nucleophile (Donor).
- Product: $\alpha,\beta$-Unsaturated Ketone (often called Chalcones).
Example: Synthesis of Chalcone
Reaction between Benzaldehyde and Acetophenone:
[attachment_0](attachment) $$ C_6H_5CHO + C_6H_5COCH_3 \xrightarrow{OH^-} C_6H_5CH=CH-COC_6H_5 \text{ (Benzalacetophenone)} $$3. Comparison Table
| Feature | Claisen Condensation | Claisen-Schmidt |
|---|---|---|
| Substrate | Esters | Aldehyde + Ketone |
| Base | Sodium Alkoxide ($RONa$) | Dilute $NaOH$ or $KOH$ |
| Product | $\beta$-Keto Ester | $\alpha,\beta$-Unsaturated Carbonyl |
| Mechanism Type | Nucleophilic Acyl Substitution | Aldol Addition + Elimination |
4. Dieckmann Condensation
Intramolecular Claisen
When a molecule contains two ester groups separated by 4 or 5 carbons (e.g., 1,6-diester), it undergoes Intramolecular Claisen Condensation to form a 5 or 6-membered cyclic $\beta$-keto ester. This is called the Dieckmann Condensation.
Reaction Mastery Quiz
Test your concepts on Claisen & Claisen-Schmidt reactions. 10 MCQs with explanations.
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