Explain the mechanism of esterification carboxylic acids.

Explanation

Mechanism of estrification of carboxylic acids : It is a kind of nucleophilhc acyl substitution. The mechanism of esterification involves the following step:
Step I. Protonation of the carbonyl group.
In presence of mineral acids (conc. `H_(2)SO_(4) or HCl`) the carbonyl oxygen of carboxylic acid accepts a proton to,form protonated carboxylic acid (I).

Step II. Nucleophilic attack by the alcohol molecule
The electron rich oxygen atom of alcohol molecule attaches itself at positively charged carbon atom to form tetrahedral intermediate (II).

Step Ill. Transfer of proton.
From the resulting intermediate, a proton shifts to -OH and forms another tetrahedral intermediate (Ill).

step IV. Loss of water molecule
The intermediate (III) loses a molecule of water to afford protonated ester (IV).

Step v. Loss of proton.
The protonated ester finally loses a proton to form an ester

The mechanism is supported by the isotropic tracer studies that esterification involves the cleavage of H of O- H bond of alcohols and -OH of - CO- OH bond of carrboxylic acids as suggested by the above mechanism. For example, when acetic acid is esterified with isotopically labelled methanol `(CH_(3) ""^(18)OH)`, the ester (i.e., methyl acetate) thus formed was found to contain `""^(18)O` while water formed did not have any isotopic oxygen.