o Fischer esterification any Fischer-Speier esterification is a carboxylic esterification reaction that uses an inorganic acid as a catalyst, mainly sulfuric acid (HtwoSO4).
We will now review the structure of carboxylic esters and the general characteristics of esterification reactions, and then explain the details of Fischer-Speier esterification.
esters These are compounds derived from the reaction between an oxyacid and an alcohol. The reactions where esters are formed are known as esterification reactions.
The most common esters are carboxylic acid esters, although there are esters of virtually any inorganic oxyacid, such as carbonic esters and phosphoric esters.types of esters
the carboxylic esters They are formed by the reaction between a carboxylic acid and an organic alcohol. For example, esterification between fatty acids and glycerol forms triglycerides.
The general structure of carboxylic esters is R-COOR’where R and R’ are residues of organic molecules.
The esterification reaction is a form of condensation reaction in which water. The carboxylic acid contributes a -COOH group (carboxyl group). The other contributes an -OH group (hydroxyl group).
Both functional groups are combined, a water molecule is released (HtwoO) and the -COO- group is formed (ester group) which joins the two molecules.General scheme of an esterification reaction
o Fischer-Speier esterification It is a carboxylic esterification reaction that uses an acid catalyst. This reaction was described in 1895 by Emil Fischer and Arthur Speier.
The acid catalyst most used in Fischer’s esterification is sulfuric acid (HtwoSO4).Fischer-Speier esterification scheme
Fischer esterification mechanism
The carboxylic acids they are generally very weak acids, so they have little tendency to give up protons (H+) that they lose in the esterification.
A stronger acid, such as sulfuric acid, behaves as a proton donor and facilitates the esterification reaction.
First, sulfuric acid injects a proton into the carboxyl group:R-COOH + H2SO4 → R-C+(OH)2 + HSO4-
The alcohol molecule, R’-OH, has an electron-rich oxygen atom, is attracted to the protonated carboxylic structure, and the following complex is formed:R-C+(OH)2 + R’-OH → R-COH(O+H2)-OR’
This complex is not very stable and stabilizes towards the most energetically favorable molecule, which is the ester, releasing a water molecule and a proton (H+). The proton is then used to regenerate sulfuric acid:R-COH(O+H2)-OR’ + HSO4- → H2SO4 + H2O + R-COO-R’ Scheme of the Fischer-Speier esterification mechanism of action
Sulfuric acid as a catalyst
As described in the Fischer esterification mechanism, sulfuric acid is regenerated and is not consumed in the reaction. This causes sulfuric acid to act as a catalyst rather than a reactant.
Sulfuric acid-catalyzed esterification reactions are useless when tertiary alcohols (hydroxyl group attached to a three-carbon chain) are involved.
These alcohols in the presence of sulfuric acid can easily undergo dehydration. no ester formation.
For example, isobutanol, with the formula (CH3)twoCHCHtwoOH, in the presence of sulfuric acid undergoes dehydration and isobutylene, with the formula (CH3)twoC=CHtwo, is formed.(CH3)2CHCH2OH + H2SO4 → (CH3)2C=CH2 + H2O + HSO4-
Therefore, Fischer esterification with sulfuric acids or other strong acids is not a viable method to synthesize tertiary esters.