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Esterification:

Fischer esterification is an example of nucleophilic acyl substitution based on the electrophilicity of the carbonyl carbon and the nucleophilicity of an alcohol. However, carboxylic acids tend to be less reactive than esters as electrophiles. Additionally, in dilute neutral solutions they tend to be deprotonated anions (and thus unreactive as electrophiles). Though kinetically very slow without any catalysts (most esters are metastable), pure esters will tend to spontaneously hydrolyse in the presence of water, so when carried out "unaided", high yields for this reaction are quite unfavorable.

Several steps can be taken to turn this unfavorable reaction into a favourable one.[4]

The reaction mechanism for this reaction has several steps:

1.	Proton transfer from acid catalyst to carbonyl oxygen increases electrophilicity of carbonyl carbon.

2.	The carbonyl carbon is then attacked by the nucleophilic oxygen atom of the alcohol

3.	Proton transfer from the oxonium ion to a second molecule of the alcohol gives an activated complex

4.	Protonation of one of the hydroxy groups of the activated complex gives a new oxonium ion.

5.	Loss of water from this oxonium ion and subsequent deprotonation gives the ester. A generic mechanism for an acid Fischer esterification is shown below.

https://en.m.wikipedia.org/wiki/File:Fischer_esterification_mechanism.svg￼

Another mechanism can also be happen like this:

https://commons.wikimedia.org/wiki/File:Esterification_2nd_mechanism.png