Esterification reaction

Esters are chemical compounds usually characterized by a pleasant smell that often find application in cosmetic companies that produce fragrances. No matter the dimension and complexity of the molecule, esters are structurally formed by a carbonyl center with one or two single bonds to alkoxy groups.

The synthetic reaction to produce them is generally called esterification and can be performed with different reagents, depending on the needs and nature of the starting material. The Fisher esterification, one of the most used, involves a carboxylic acid and an alcohol as reagents, with the formation of water as a side product. Furthermore, as the reaction is rather slow, an acid that plays a catalytic effect is often added to speed up the reaction.

The mechanism of how an esterification reaction occurs is complicated and involves different stages to turn the acid into an ester:

  • Protonation: the carboxylic acid takes a proton from the catalyst attaching it to the double-bonded oxygen and delocalizing a positive charge.

  • Nucleophilic addition: the delocalized positive charge rests mainly on the central carbon, attracting one of the lone pairs on the oxygen of the alcohol.

  • Proton transfer: the newly attached alcohol is positively charged and therefore transfers one of its hydrogens to another of the hydroxyl groups thanks to an unreacted alcohol molecule in solution.

  • Elimination of water: a molecule of water is lost and the remaining positive charge is stabilized through delocalization, forming different resonance structures.

The mechanism of esterification reactions is based on different steps. It starts with a protonation, where the carboxylic acid gains an hydrogen from the acid that works as catalyst. Then a nucleophilic reaction takes place, where the water attaches itself to the central carbon of the positively charged carboxylic acid. These steps are followed by a proton transfer and elimination of water steps, where the double bond to the oxygen is recreated. A resonance stage deprotonates the positively charged oxygen by releasing the catalytic acid in solution and creating the ester.

Figure 1: Fischer esterification mechanism.