14.19
Esters to Carboxylic Acids: Acid-Catalyzed Hydrolysis
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Q1: What happens to an ester during acid-catalyzed hydrolysis?
During acid-catalyzed hydrolysis, an ester is broken down in the presence of aqueous acid to form a carboxylic acid and an alcohol. This is a reversible reaction that proceeds through nucleophilic acyl substitution, where the acid catalyst activates the ester carbonyl carbon, making it more susceptible to nucleophilic attack by water molecules.
Q2: How does the acid catalyst increase the reaction rate in ester hydrolysis?
The acid catalyst protonates the carbonyl oxygen of the ester, making the carbonyl carbon significantly more electrophilic and susceptible to nucleophilic attack. Additionally, the catalyst decreases the basicity of the leaving group, facilitating its departure. These effects accelerate the formation of the tetrahedral intermediate and overall reaction rate.
Q3: What is the role of the tetrahedral intermediate in ester hydrolysis?
The tetrahedral intermediate forms when water attacks the activated carbonyl carbon. After deprotonation, this neutral intermediate is then protonated at the alkoxy oxygen, converting the alkoxy group into a better leaving group. The carbonyl group is subsequently re-formed as the alcohol departs, leading to carboxylic acid formation.
Q4: How is acid-catalyzed ester hydrolysis related to Fischer esterification?
Acid-catalyzed hydrolysis is precisely the reverse of Fischer esterification. While Fischer esterification combines a carboxylic acid and alcohol to form an ester under acidic conditions, hydrolysis breaks down the ester back into carboxylic acid and alcohol. Both reactions are reversible and proceed through similar mechanistic intermediates.
Q5: Why is the alkoxy group converted into a better leaving group during hydrolysis?
The alkoxy group is protonated by the acid catalyst, converting it from a poor leaving group into a better one. This protonation reduces the negative charge on the oxygen, making it easier for the C-O bond to break and the alcohol to depart as a neutral molecule, facilitating carbonyl group re-formation.
Q6: What is regenerated at the end of acid-catalyzed ester hydrolysis?
The acid catalyst is regenerated when the final carboxylic acid product is deprotonated. This regeneration allows the catalyst to participate in additional reaction cycles without being consumed, making it a true catalyst that enables the hydrolysis of multiple ester molecules.
Q7: What are the main products formed from ester hydrolysis under acidic conditions?
The main products are a carboxylic acid and an alcohol. The carboxylic acid forms from the acyl portion of the ester, while the alcohol comes from the alkoxy group. The reaction requires excess water and proceeds in a reversible manner under acidic aqueous conditions.