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

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