Mechanism of Acid-Catalyzed Esterification

An important qnestion about the mechanism of acid-catalyzed esterification concerns the origin of the aUcoxy oxygen. For example, does the methoxy oxygen in methyl benzoate come from methanol, or is it derived from benzoic acid  

Is this the oxygen originally present in henzoic acid, or is it the oxygen of methanol  

The answer to this question is critical becanse it teUs ns whether the carbon-oxygen bond of the alcohol or a carbon-oxygen of the carboxylic acid is broken dnring the esterification. 

A clear-cut answer was provided by Irving Roberts and Harold C. Urey of Columbia University in 1938. They prepared methanol that had been enriched in the mass-18 isotope of oxygen. When this sample of methanol was esterifled with benzoic acid, the methyl benzoate product contained all the label that was originally present in the methanol. 

The results of the Roberts-Urey experiment tell us that the C—O bond of the alcohol is preserved during esterification. The oxygen that is lost as a water molecule must come from the carboxylic acid. 

In this equation, the red 0 signifies oxygen enriched in its mass-18 isotope analysis of isotopic enrichment was performed by mass spectrometry. 

Mechanism 18.1 is consistent with these facts. The six steps are best viewed as a combination of two distinct stages. Formation of a tetrahedral intermediate characterizes 

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Section 19 14 The mechanism of acid catalyzed esterification involves some key fea tures that are fundamental to the chemistry of carboxylic acids and then-derivatives... 

FIGURE 19.7 The mechanism of acid-catalyzed esterification of benzoic acid with methanol. 

The Mechanism of Acid-Catalyzed Esterification Nucleophilic Acyl Substitution... 

Fig. 9. Mechanism of acid catalyzed esterification of carboxylic acid...

The mechanism of acid-catalyzed esterification involves two stages. The first is formation of a tetrahedral intermediate by nucleophilic addition of the alcohol to the carbonyl group and is analogous to acid-catalyzed acetal and ketal formation of aldehydes and ketones. The second is dehydration of the tetrahedral intermediate. 

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