Ch. 24 - Amino Acids, Peptides, and Proteins

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 24 - Amino Acids, Peptides, and Proteins

Problem 24a

Chapter 24, Problem 24a

Propose a mechanism for the coupling of acetic acid and aniline using DCC as a coupling agent.

Verified step by step guidance

Identify the role of DCC (dicyclohexylcarbodiimide) in the reaction: DCC is a coupling agent that activates the carboxylic acid group of acetic acid, making it more reactive toward nucleophilic attack by the amine group of aniline.

Step 1: Activation of acetic acid by DCC. The lone pair of electrons on the oxygen atom of the carboxylic acid group in acetic acid attacks the carbon atom of the carbodiimide group in DCC, forming an O-acylisourea intermediate. This intermediate is highly reactive and facilitates the coupling reaction.

Step 2: Nucleophilic attack by aniline. The lone pair of electrons on the nitrogen atom of aniline attacks the carbonyl carbon of the O-acylisourea intermediate, forming a tetrahedral intermediate.

Step 3: Collapse of the tetrahedral intermediate. The tetrahedral intermediate collapses, leading to the formation of the amide bond between acetic acid and aniline, and dicyclohexylurea (DCU) as a byproduct.

Step 4: Final product formation. The reaction results in the formation of acetanilide (an amide) as the final product, along with DCU, which can be removed by filtration due to its insolubility in many solvents.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Coupling Reactions

Coupling reactions involve the formation of a new bond between two molecules, typically involving a nucleophile and an electrophile. In this context, acetic acid acts as a carboxylic acid nucleophile, while aniline serves as an amine nucleophile. Understanding the nature of these reactants and their reactivity is crucial for proposing a mechanism.

DCC (Dicyclohexylcarbodiimide)

DCC is a commonly used coupling agent in organic synthesis that facilitates the formation of amide bonds between carboxylic acids and amines. It activates the carboxylic acid by converting it into a more reactive intermediate, allowing for the nucleophilic attack by the amine. Familiarity with DCC's role and mechanism is essential for understanding how it aids in the coupling of acetic acid and aniline.

Mechanism of Amide Formation

The mechanism of amide formation typically involves the nucleophilic attack of an amine on the activated carboxylic acid. In the presence of DCC, the carboxylic acid forms an O-acylurea intermediate, which is then attacked by the amine, leading to the formation of the amide bond and the release of dicyclohexylurea as a byproduct. Understanding this stepwise process is vital for accurately proposing the overall reaction mechanism.

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