Textbook Question
Predict the product of the following reactions.
(b)
25. Condensation Chemistry
Robinson Annulation
4:12 minutesProblem 58a
Textbook Question
The base-catalyzed reaction of an aldehyde (having no α hydrogens) with an anhydride is called the Perkin condensation. Propose a mechanism for the following example of the Perkin condensation. (Sodium acetate serves as the base.)
Verified step by step guidance1
Step 1: The reaction begins with the deprotonation of the anhydride by sodium acetate (CH₃CO₂Na), which acts as the base. This generates an enolate ion from the anhydride. The enolate ion is stabilized by resonance between the oxygen and the carbonyl group.
Step 2: The enolate ion then performs a nucleophilic attack on the carbonyl carbon of the aldehyde (benzaldehyde). This forms a tetrahedral intermediate, which subsequently collapses to eliminate acetate (CH₃CO₂⁻), regenerating the base.
Step 3: The resulting intermediate undergoes an aldol condensation. A proton is abstracted from the α-carbon of the intermediate, forming a new enolate ion. This enolate ion then eliminates a molecule of acetic acid (CH₃COOH), forming the α,β-unsaturated product.
Step 4: The reaction mixture is subjected to acidic workup (H₃O⁺), which ensures the protonation of any remaining intermediates and stabilizes the final product. This step also helps to isolate cinnamic acid as the major product.
Step 5: The final products of the reaction are cinnamic acid (an α,β-unsaturated carboxylic acid) and acetic acid (CH₃COOH). The mechanism highlights the role of sodium acetate as the base and the aldol condensation pathway in forming the conjugated double bond in cinnamic acid.
Verified video answer for a similar problem:This video solution was recommended by our tutors as helpful for the problem above
Video duration:
4mWas this helpful?
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Perkin Condensation
The Perkin condensation is a reaction between an aldehyde and an anhydride, typically catalyzed by a base, to form α,β-unsaturated carboxylic acids. This reaction is significant in organic synthesis for constructing complex molecules. In this process, the aldehyde must lack α-hydrogens, which prevents self-condensation and allows for the formation of a specific product.
Recommended video:
Guided course
05:53
Condensation Reactions
Mechanism of Base-Catalyzed Reactions
Base-catalyzed reactions involve the use of a base to facilitate the reaction by deprotonating a substrate, thereby increasing its nucleophilicity. In the context of the Perkin condensation, sodium acetate acts as the base, which helps in the formation of the nucleophile that attacks the anhydride. Understanding this mechanism is crucial for predicting the reaction pathway and the formation of intermediates.
Recommended video:
Guided course
01:57Base Catalyzed
Role of Sodium Acetate
Sodium acetate serves as a mild base in the Perkin condensation, promoting the reaction without overly aggressive conditions. It deprotonates the aldehyde, generating a more reactive enolate ion that can effectively attack the anhydride. This role is essential for the successful progression of the reaction and the formation of the desired α,β-unsaturated product.
Recommended video:
Guided course
04:29Acetal Protecting Group
Related Videos
Related Practice