Everything about Fischer Esterification totally explained
Fischer esterification or
Fischer-Speier 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. The reaction was first described by
Emil Fischer and
Arthur Speier in 1895.
Most carboxylic acids are suitable for the reaction, but the alcohol should generally be a primary or secondary alkyl. Tertiary alcohols are prone to elimination, and phenols are usually too unreactive to give useful yields. Commonly used catalysts for a Fischer esterification include
sulfuric acid,
tosic acid, and
lewis acids such as
scandium(III) triflate. The reaction is often carried out without a solvent (particularly when a large reagent excess (for example of MeOH is used) or in a non-polar solvent (for example
toluene) to facilitate the
Dean-Stark method. Typical reaction times vary from 1-10 hours at temperatures of 60-110°C, but in small lab experiments, the presence of esters can be detected within minutes.
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 for example by a large excess of one of the reagents, or by the removal of water (for example by
Dean-Stark distillation, the use of
molecular sieves, or the use of a stoichiometric quantity of concentrated sulfuric acid as the catalyst). Solid phase acids (such as the polymer
Nafion and
ion exchange resins can also be used.
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
- Protonation of one of the hydroxyl groups of the activated complex gives a new oxonium ion.
- Loss of water from this oxonium ion and subsequent deprotonation gives the ester.
A generic mechanism for an acid Fischer esterification is shown below using acetic acid.
An example of an esterification is in the formation of
benzocaine.
Further Information
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