Reaction mechanisms of esterification

In the esterification of a carboxylic acid with an alcohol, reaction occurs by acyl-oxygen (1) or alkyl-oxygen (2) heterolysis  

Ingold [5] showed that acid-catalysed esterification involves three possible mechanisms, denoted as A c-Aac2 and A li, where A is the substrate (conjugate acid of the carboxylic acid), AC and AL denote acyl and alkyl bond heterolysis, and the numbers represent the molecularity of the rate-determining step. 

The esterification reaction is mainly bimolecular, and goes via acyl-oxygen fission, where the rate-limiting step is the attack of alcohol on the protonated carboxylic acid. Ingold proposed the following schemes  

The carbon atom becomes tetrahedrally bonded, and for this reason retardation of the reaction rate may occur where —R and —R are bulky groups which may interact sterically. 

This mechanism is common for tertiary alcohols, and explains the formation of racemized esters when ester-ifying with an optically active alcohol. This reaction is slow for primary alcohols. The alcohol is first protonated and then loses water to form a carbonium ion, which reacts rapidly with the acid. 

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Fig. 3. Reaction mechanism of esterification of 2-hydroxyalkylamides via an oxazolinium intermediate...

Reaction mechanisms of esterification 12 2.3.2 Methyl esters with ... 

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