Textbook Question
Acid-catalyzed transesterification:
Complete the mechanism for this acid-catalyzed transesterification by drawing out all the individual steps. Draw the important resonance contributors for each resonance-stabilized intermediate.
22. Carboxylic Acid Derivatives: NAS
Transesterification
2:51 minutesProblem 25b
Textbook Question
What products are obtained from the following reactions? b. phenyl acetate + excess ethanol + HCl
Verified step by step guidance1
Step 1: Recognize the type of reaction. This is an acid-catalyzed esterification (Fischer esterification) or transesterification reaction, where phenyl acetate (an ester) reacts with excess ethanol in the presence of an acid catalyst (HCl).
Step 2: Understand the mechanism. In the presence of HCl, the carbonyl oxygen of the ester (phenyl acetate) gets protonated, making the carbonyl carbon more electrophilic and susceptible to nucleophilic attack by ethanol.
Step 3: Identify the nucleophile and electrophile. Ethanol acts as the nucleophile, attacking the electrophilic carbonyl carbon of phenyl acetate, leading to the formation of a tetrahedral intermediate.
Step 4: Follow the reaction pathway. The tetrahedral intermediate undergoes proton transfers and elimination of the leaving group (phenol, C6H5OH), resulting in the formation of a new ester, ethyl acetate (CH3COOCH2CH3).
Step 5: Summarize the products. The reaction produces ethyl acetate (the new ester) and phenol as the byproduct. The excess ethanol ensures the reaction proceeds to completion by shifting the equilibrium toward product formation.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Ester Hydrolysis
Ester hydrolysis is a reaction where an ester reacts with water or an alcohol in the presence of an acid or base, leading to the formation of an alcohol and a carboxylic acid. In this case, phenyl acetate undergoes hydrolysis in the presence of excess ethanol and HCl, resulting in the formation of phenol and acetic acid.
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Fischer Esterification
Fischer esterification is a process where a carboxylic acid reacts with an alcohol to form an ester and water, typically catalyzed by an acid. In the context of the given reaction, the excess ethanol can also act as a reactant, potentially leading to the formation of ethyl acetate alongside the hydrolysis products.
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Role of Catalysts in Reactions
Catalysts, such as HCl in this reaction, are substances that increase the rate of a chemical reaction without being consumed in the process. In ester hydrolysis and Fischer esterification, HCl serves to protonate the carbonyl oxygen of the ester, making it more susceptible to nucleophilic attack by water or ethanol, thus facilitating the reaction.
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