Fischer Esterification

Direct Acylation s of alcohols with carboxylic acids is preferred over acylations with Anhydride s (poor Atom Economy ) or Acid Chloride s (moisture sensitive). The main disadvantage of direct acylation is the unfavorable Chemical Equilibrium that must be remedied by removal of water either by Dean-Stark Distillation or the use of Molecular Sieve s. In one study it is found that Tetrabutylammonium Tribromide (TBATB) is a very effective Catalyst . For example the acylation of 3-phenyl propanol with Glacial Acetic Acid and TBATB at reflux generates the ester in 15 minutes in a 95% Chemical Yield without the need to remove water. It is believed that Hydrobromic Acid released by TBATB protonates the alcohol over the acid making the carboxylate the actual nucleophile in a reversal of the standard esterification mechanism.

MECHANISM

The Reaction Mechanism for this reaction has several steps:
#Proton transfer from acid catalyst to Carbonyl oxygen increases Electrophilicity of carbonyl carbon.
#The carbonyl carbon is then attacked by the Nucleophilic oxygen atom of the alcohol.
#leading to the formation of an Oxonium ion.
#Proton transfer from the Oxonium ion to a second molecule of the alcohol gives an activated complex
#Proton transfer from one of the hydroxyl groups of the activated complex gives a new Oxonium ion.
#Loss of water from this oxonium ion gives the Ester and Water

An example of an esterification is in the formation of Benzocaine .

REFERENCES

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Fischer Esterification