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Ch. 20 - Carboxylic Acids
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 20 - Carboxylic AcidsProblem 16a,b
Chapter 20, Problem 16a,b

The mechanism of the Fischer esterification was controversial until 1938, when Irving Roberts and Harold Urey of Columbia University used isotopic labeling to follow the alcohol oxygen atom through the reaction. A catalytic amount of sulfuric acid was added to a mixture of 1 mole of acetic acid and 1 mole of special methanol containing the heavy 18O isotope of oxygen. After a short period, the acid was neutralized to stop the reaction, and the components of the mixture were separated.

(a) Propose a mechanism for this reaction.
(b) Follow the labeled 18O atom through your mechanism, and show where it is found in the products.

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Step 1: Begin by understanding the Fischer esterification mechanism. This reaction involves the acid-catalyzed reaction between a carboxylic acid and an alcohol to form an ester and water. Sulfuric acid acts as a catalyst, protonating the carboxylic acid to make it more electrophilic.
Step 2: Protonation of the acetic acid occurs first. The sulfuric acid donates a proton (H⁺) to the carbonyl oxygen of acetic acid, increasing the electrophilicity of the carbonyl carbon. This step is crucial for the nucleophilic attack by the alcohol.
Step 3: The nucleophilic alcohol (CH₃¹⁸OH) attacks the carbonyl carbon of the protonated acetic acid. This forms a tetrahedral intermediate where the labeled ¹⁸O atom from the alcohol is now attached to the carbonyl carbon.
Step 4: The tetrahedral intermediate undergoes proton transfer and elimination of water. A proton is transferred to the hydroxyl group of the intermediate, converting it into a good leaving group. Water (H₂O) is eliminated, leaving behind the ester product.
Step 5: The labeled ¹⁸O atom from the alcohol is retained in the ester product (CH₃COOCH₃). The water molecule formed in the reaction does not contain the labeled oxygen, confirming the fate of the ¹⁸O atom in the mechanism.

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Key Concepts

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

Fischer Esterification

Fischer esterification is a chemical reaction that forms an ester from a carboxylic acid and an alcohol in the presence of an acid catalyst, typically sulfuric acid. The reaction is reversible and involves the nucleophilic attack of the alcohol on the carbonyl carbon of the acid, leading to the formation of an ester and water. Understanding this mechanism is crucial for predicting the behavior of the reactants and products in the reaction.
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Isotopic Labeling

Isotopic labeling is a technique used in chemistry to track the movement of atoms through a reaction by substituting a stable or radioactive isotope for a more common isotope. In this case, the heavy 18O isotope is used to trace the oxygen atom from methanol through the Fischer esterification process. This method provides insights into reaction mechanisms and helps confirm the pathways taken by specific atoms during chemical transformations.
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Reaction Mechanism

A reaction mechanism is a step-by-step description of the process by which reactants are converted into products. It outlines the individual steps, intermediates, and transition states involved in the reaction. For Fischer esterification, understanding the mechanism allows chemists to predict the outcome of the reaction, including the location of the labeled 18O atom in the final ester product, which is essential for answering the question posed.
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Related Practice
Textbook Question

Show how Fischer esterification might be used to form the following esters. In each case, suggest a method for driving the reaction to completion.

(c) ethyl phenylacetate

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Textbook Question

Show how Fischer esterification might be used to form the following esters. In each case, suggest a method for driving the reaction to completion.

(a) methyl salicylate

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Textbook Question

Show how to synthesize the following compounds, using appropriate carboxylic acids and amines.

(a)

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Textbook Question

(b) Finish the solution for Solved Problem 20-1 by providing a mechanism for the acid-catalyzed hydrolysis of ethyl formate.

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Textbook Question

The given was missing some important resonance forms of the intermediates shown in brackets. Complete this mechanism by drawing all the resonance forms of these intermediates. Do your resonance forms help to explain why this reaction occurs under very mild conditions (water with a tiny trace of acid)?

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Textbook Question

What monosaccharides are formed in a modified Kiliani–Fischer synthesis starting with each of the following monosaccharides?

b. L-threose

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