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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/07/30 17:21:38」(JST)

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In organic chemistry, transesterification /ˌtrænzˌɛstərɨfɨˈkeɪʃən/ is the process of exchanging the organic group R″ of an ester with the organic group R′ of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. The reaction can also be accomplished with the help of enzymes (biocatalysts) particularly lipases (E.C.3.1.1.3).

Transesterification: alcohol + ester → different alcohol + different ester

Strong acids catalyse the reaction by donating a proton to the carbonyl group, thus making it a more potent electrophile, whereas bases catalyse the reaction by removing a proton from the alcohol, thus making it more nucleophilic. Esters with larger alkoxy groups can be made from methyl or ethyl esters in high purity by heating the mixture of ester, acid/base, and large alcohol and evaporating the small alcohol to drive equilibrium.

Mechanism

In the transesterification mechanism, the carbonyl carbon of the starting ester (RCOOR¹) undergoes nucleophilic attack by the incoming alkoxide (R²O⁻) to give a tetrahedral intermediate, which either reverts to the starting material, or proceeds to the transesterified product (RCOOR²). The various species exist in equilibrium, and the product distribution depends on the relative energies of the reactant and product.

Applications

Polyester production

The largest scale application of transesterification is in the synthesis of polyesters.^[1] In this application diesters undergo transesterification with diols to form macromolecules. For example, dimethyl terephthalate and ethylene glycol react to form polyethylene terephthalate and methanol, which is evaporated to drive the reaction forward.

Methanolysis and biodiesel production

The reverse reaction, methanolysis, is also an example of transesterification. This process has been used to recycle polyesters into individual monomers (see plastic recycling). It is also used to convert fats (triglycerides) into biodiesel. This conversion was one of the first uses. Transesterified vegetable oil (biodiesel) was used to power heavy-duty vehicles in South Africa before World War II.

It was patented in the US in the 1950s by Colgate, though Biolipid transesterification may have been discovered much earlier. In the 1940s, researchers were looking for a method to more readily produce glycerol, which was used to produce explosives for World War II. Many of the methods used today by producers and home brewers have their origin in the original 1940s research.

Biolipid transesterification has also been recently shown by Japanese researchers to be possible using a super-critical methanol methodology, whereby high temperature, high-pressure vessels are used to physically catalyze the biolipid/methanol reaction into fatty-acid methyl esters.

High-pressure transesterification

Base-catalyzed transesterification is characterized by a negative activation volume (approx. −12 cm³) and therefore it proceeds faster under high-pressure conditions. It has been shown that amine-catalyzed alcoholysis of sterically hindered esters (e.g. protecting groups, chiral auxiliaries) proceeds rapidly at room temperature under 10 kbar pressure, giving quantitative yields.^[2]

References

^ Wilhelm Riemenschneider1 and Hermann M. Bolt "Esters, Organic" Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a09_565.pub2
^ Romanski, J.; Nowak, P.; Kosinski, K.; Jurczak, J. (Sep 2012). "High-pressure transesterification of sterically hindered esters". Tetrahedron Lett. 53 (39): 5287–5289. doi:10.1016/j.tetlet.2012.07.094.

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English Journal

Lipase-catalyzed transesterification of soybean oil and phytosterol in supercritical CO2.

Hu L1,2, Llibin S3, Li J3, Qi L3, Zhang X3, Yu D4,5, Walid E6, Jiang L7,8.
Bioprocess and biosystems engineering.Bioprocess Biosyst Eng.2015 Dec;38(12):2343-7. doi: 10.1007/s00449-015-1469-5. Epub 2015 Sep 19.
The transesterification of phytosterol and soybean oil was performed using Novozym 435 in supercritical carbon dioxide (SC-CO2). The transesterification reaction was conducted in soybean oil containing 5-25 % phytosterol at 55-95 °C and free-water solvent. The effects of temperature, reaction tim
PMID 26386863

Conversion of dried Aspergillus candidus mycelia grown on waste whey to biodiesel by in situ acid transesterification.

Kakkad H1, Khot M1, Zinjarde S1, RaviKumar A2, Ravi Kumar V3, Kulkarni BD3.
Bioresource technology.Bioresour Technol.2015 Dec;197:502-7. doi: 10.1016/j.biortech.2015.07.118. Epub 2015 Aug 28.
This study reports optimization of the transesterification reaction step on dried biomass of an oleaginous fungus Aspergillus candidus grown on agro-dairy waste, whey. Acid catalyzed transesterification was performed and variables affecting esterification, viz., catalyst methanol and chloroform conc
PMID 26362462

Transesterification of activated sludge in subcritical solvent mixture.

Tran-Nguyen PL1, Go AW2, Ismadji S3, Ju YH4.
Bioresource technology.Bioresour Technol.2015 Dec;197:30-6. doi: 10.1016/j.biortech.2015.08.033. Epub 2015 Aug 20.
Most previous studies reported in literature on biodiesel production from sludge were performed by acid catalyzed transesterification that required long reaction time (about 24h) and high methanol loading. The objective of this study was to investigate the in situ transesterification of sludge in su
PMID 26318244