Structural Biochemistry/Organic Chemistry/Method of Trans Esterification

Trans Esterifcation

This is an organic reaction for the preparation of esters. Instead of Fischer esterification (ester synthesis through carboxylic acid and alcohol) trans-esterification method allows R-substituent to exchange from an alcohol to the ester and vice versa. This is made possible through the use of acid or base catalysts.

Acid Catalyzed Trans-Esterification

Strong acids such as sulfuric, phosphoric, and hydrochloric acids can be used to catalyze the reaction.

Mechanism: In acidic conditions, protonation occurs at the carbonyl and enables a partial positive carbon. This is where the alcohol attaches and then deprotonated by another alcohol. The formation of a tetrahedral complex occurs and an -OR’ group will be protonated due to acidic conditions. It will leave and form a new ester with the R substituent from the alcohol.

Base Catalyzed Trans-Esterification

Bases can be used to catalyze the reaction (for example, K2CO3 or NaOH).

Mechanism: In basic conditions, deprotonation of the alcohol occurs and enables a strong nucleophile to attack the partial positive carbonyl carbon. In response to the new attachment, electrons move from the double bonded oxygen to form a negative charge on the oxygen. To stabilize the tetrahedral complex, the lone pairs of electrons reform and force the OR’ group to leave. The remaining product is an ester with a new R-substituent, and a negatively charged OR’.

Importance of Trans-Esterification

Biodiesel is an ester product that is made possible by trans-esterification. It starts out with triglyceride and excess methanol to form methyl-esters and glycerol. This process is used to make bio-fuel. However, because it is found in oils of food products, it isn’t the best way to mass-produce because of food scarcities. It is a good alternative to fossil fuel and a lot of fast food restaurants convert their cooking oil into useable bio-fuel.

References

Otera, J;Transesterification.Chem.Rev.1993. 93,1449-1470 http://pubs.acs.org/doi/pdf/10.1021/cr00020a004

"Biodiesel Synthesis." (n.d.): 1-3. Web. http://faculty.northseattle.edu/tfurutani/che238_2006/biodiesel.pdf