Transesterification is the process of exchanging the organicfunctional group R″ of anester with the organic group R' of analcohol. These reactions are oftencatalyzed by the addition of anacid orbase catalyst.^[1] Strong acids catalyze the reaction by donating aproton to thecarbonyl group, thus making it a more potentelectrophile. Bases catalyze the reaction by removing a proton from the alcohol, thus making it morenucleophilic. The reaction can also be accomplished with the help of enzymes, particularlylipases (one example is the lipase E.C.3.1.1.3^[2]).

If the alcohol produced by the reaction can be separated from the reactants by distillation this will drive theequilibrium toward the products. This means that esters with largeralkoxy groups can be made from methyl or ethyl esters in high purity by heating the mixture of ester, acid/base, and large alcohol.

In the transesterification mechanism, the carbonyl carbon of the starting ester reacts to give atetrahedral intermediate, which either reverts back 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. Depending on reaction conditionsester hydrolysis and/oresterification will also occur, which results in some amount of free carboxylic acid being present.

The largest scale application of transesterification is in the synthesis ofpolyesters.^[3] In this application, diesters undergo transesterification with diols to form macromolecules. For example,dimethyl terephthalate andethylene glycol react to formpolyethylene terephthalate andmethanol, which is evaporated to drive the reaction forward.

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

It waspatented in the US in the 1950s byColgate, thoughbiolipid transesterification may have been discovered much earlier. In the 1940s, researchers were looking for a method to more readily produceglycerol, which was used to produceexplosives for World War II. Many of the methods used today by producers have their origin in the original 1940s research.

Biolipid transesterification has also been recently shown by Japanese researchers to be possible using asupercritical methanol methodology, whereby high temperature, high-pressure vessels are used to physically catalyze the biolipid/methanol reaction into fatty-acid methyl esters.^[4]

Fat interesterification is used in thefood industry to rearrange thefatty acids oftriglycerides in ediblefats andvegetable oils. For example, a solid fat with mostly saturated fatty acids may be transesterified with a vegetable oil having high unsaturated acid contents, to produce a spreadable semisolid fat whose molecules have a mix both kinds of acids.

Transesterification is used to synthesizeenol derivatives, which are difficult to prepare by other means.Vinyl acetate, which is cheaply available, undergoes transesterification, giving access tovinyl ethers:^[5][6]

ROH +AcOCH=CH
₂ ⟶ROCH=CH
₂ + AcOH

The reaction can be effected with high enantioselectivity when mediated with alipase.^[7]

• Biodiesel production
• Otera's catalyst
• Transalkylation
• Transamidification
• Cocaethylene