Fischer esterification
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Fischer esterification is a special type of esterification and the process of forming an ester by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst. Commonly used catalysts for a Fischer esterification include sulfuric acid or lewis acids such as scandium(III) triflate.
Direct acylations of alcohols with carboxylic acids is preferred over acylations with anhydrides (poor atom economy) or acid chlorides (moisture sensitive). The main disadvantage of direct acylation is the unfavorable chemical equilibrium that must be remedied by an excess of carboxylic acid, or the removal of water either by Dean-Stark distillation, or the use of molecular sieves. In one study [1] 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.
[edit] 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.
[edit] References
- ^ Tetrabutylammonium tribromide mediated condensation of carboxylic acids with alcohols Sarala Naik, Veerababurao Kavala, Rangam Gopinath, and Bhisma K. Patel Arkivoc 2006 (i) 119-127 Online Article
