A particular kind of esterification by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst is Fischer esterification or Fischer-Speier esterification. Esterification occurs when an alcohol reacts with a carboxylic acid. Only in the presence of an acid catalyst and heat will this reaction occur. To extract the -OH from the carboxylic acid requires a lot of energy, so a catalyst and heat are required to generate the necessary energy.
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Arthur Speir and Emil Fischer first described the reaction in 1895. For the reaction, most carboxylic acids are appropriate, but the alcohol should usually be primary or secondary. Tertiary alcohols are vulnerable to being removed.
As the overall reaction is reversible, the Fischer Esterification Process must involve the continuous removal of water from the system or the use of a significant excess of alcohol. The process starts with the carboxylic acid protonation of the carbonyl group, which is then attacked by alcohol. The transfer of protons and the resulting release of water results in the intermediation of the oxonium ion. Sulfuric acid and p-toluene sulfonic acid are widely used catalysts for Fischer esterification.
The Mechanism of Fischer Esterification
Following the Le Châtelier theory, the reaction is always carried out in large excess of alcohol to change the equilibrium towards the ester product. The removal of water from the reaction mixture is another method for this, depending on the physical properties of the materials.
Both steps in Fischer esterification are reversible and in general, a slow reaction using strong acids such as sulfuric or phosphoric acids is carried out in reflux.
A more reactive electrophile results from the addition of a proton (e.g.: p-TsOH, H2SO4) or a Lewis acid. The alcohol nucleophilic attack creates a tetrahedral intermediate in which two identical hydroxyl groups exist. After a proton shift (tautomerism) to provide water and the ester, one of these hydroxyl groups is removed.
At first, The carbonyl oxygen is protonated by the acid catalyst and activated by ethanol in the direction of a nucleophilic attack. Next, The alcohol infects the carbonyl with a nucleophilic attack. The bond with the carbonyl carbon forms a lone pair of electrons from the oxygen atom of the alcohol, breaking its pi bond with the other oxygen. The electrons of the pi bond pass up to the oxygen and neutralize their positive charge. This results in an ion of oxonium.
Advantages of The Fischer Esterification Mechanism
Compared with other esterification methods, the primary benefits of Fischer esterification are based on its relative simplicity. When acid-sensitive functional groups are not a concern, clear acidic conditions can be used; sulfuric acid can be used; weaker acids can be used with a tradeoff of longer reaction times. Because the reagents used are’ direct,’ in terms of waste products and the harmfulness of the reagents, there is a less environmental effect.
As compared with ester synthesis through acyl chlorides, the chemicals used and by-products released are non-toxic to the environment. A basic laboratory setup can also be used to conduct the reaction without the need for stringent safety measures or access to less common synthesis reagents.
Disadvantages of The Fischer Esterification Mechanism
Its thermodynamic reversibility and relatively slow reaction speeds are the primary drawbacks of Fischer esterification paths, frequently on the scale of several hours to years, depending on the reaction conditions. If there are other functional groups susceptible to strong acid, workarounds to this may be inconvenient, in which case other catalytic acids can be picked. The substance can be distilled rather than water if the product ester has a lower boiling point than either water or reagents.
Esterification by Fischer is not the best method of generating phenol esters. Alcohols are not the largest nucleophiles, and under such strong acidic conditions, where a considerable portion of the OH groups are in the conjugate acid form, this will be particularly true. Another drawback of Fischer esterification is that in the presence of strong acids, tertiary alcohols undergo rapid” dehydration. In these cases, alternate methods can be used to prepare the desired ester, such as the higher reactivity of acid chlorides and the carboxylate salts, discussed above.
Examples of the Fischer Esterification Mechanism
An example of acid-catalyzed esterification is the normal esterification that occurs in wines and other alcoholic drinks during the ageing process. For the esterification reaction, graphene oxide is an important and reusable acid catalyst. A large variety of aliphatic and aromatic acids and alcohols is compatible and produced good yields for the corresponding products. Protic Acid Immobilized as an extremely effective Recyclable Catalyst on Solid Support.
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