Study Notes On The Doebner Modification (Chemical Science) - Download PDF!

By Astha Singh|Updated : June 17th, 2022

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Short Note On The Doebner Modification

It is an extension of the Knoevenagel reaction.

When one of the electrons withdrawing groups on nucleophile is a carboxylic acid, i.e., if malonic acid is used in place of the active methylene group and it reacts with an aldehyde in presence of a mild base, the condensation reaction occurs and the product formed undergoes decarboxylation (loss of CO2) and yields α, β-unsaturated carboxylic acid.

This process is termed the Doebner modification.

Here, the base used is Pyridine.

Reaction mechanism-

Base abstracts the active methylene hydrogen atom resulting in enolate ion(I). The enolate formed in this case is a dianion and is quite stable due to extensive delocalization and the intramolecular hydrogen bond (II).

The dianion attacks as a nucleophile to the electrophilic carbonyl carbon of the aldehyde, and after proton abstraction from the medium (III), the aldol (IV) is formed (monocarboxylate in this basic solution).

From the aldol (IV), the next step to take place is decarboxylation and dehydration. The decarboxylation and dehydration occur via a cyclic transition state (V). Note that ideally, CO2 from any of the two —COOH groups could happen, which could give either E or Z double bond in the product. But since the product has E (or trans) geometry, that transition state is involved which has lower energy leading to a more stable E alkenoic acid (VI) as the product.

The Doebner modification of the Knoevenagel condensation. Acrolein and malonic acid react in pyridine to give trans-2,4-pentadienoic acid with the loss of carbon dioxide.

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