The Mechanism of Acid-Catalyzed Esterification Nucleophilic Acyl Substitution

The Mechanism of Acid-Catalyzed Esterification Nucleophilic Acyl Substitution 

We can ask the following simple mechanistic question about Fischer esterification Is the water molecule formed from the hydroxyl group of the acid and the hydrogen of the alcohol (as shown in color in eq. 10.17) or from the hydrogen of the acid and the hydroxyl group of the alcohol This question may seem rather trivial, but the answer provides a key to understanding much of the chemistry of acids, esters, and their derivatives. 

This question was resolved using isotopic labeling. For example, Fischer esterification of benzoic acid with methanol that had been enriched with the isotope of oxygen gave labeled methyl benzoate.  

How can we explain this experimental fact A mechanism consistent with this result is as follows (the oxygen atom of the alcohol is shown in color so that its path can be traced)  

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