Transesterifikaation

Transesterifikaation, also known as transesterification, is a chemical reaction in which an ester exchanges its alkoxy group with another alcohol, yielding a different ester and a new alcohol. The reaction is widely used to modify ester structures and to convert fats, oils, or esters into more useful esters. It can be catalyzed by acids or bases and proceeds via nucleophilic acyl substitution.

Mechanism: In base-catalyzed transesterification, an alkoxide species attacks the carbonyl carbon of the ester, forming a tetrahedral intermediate and releasing the original alkoxide as a leaving group; the process repeats until the new ester is formed. In acid-catalyzed variants, protonation of the carbonyl increases electrophilicity and facilitates alcohol addition and ester exchange. Equilibria favor the product ester when the reaction mixture includes excess alcohol or is driven by removing by-product alcohol.

Applications: The most prominent industrial use is production of biodiesel, where triglycerides from fats or oils react with methanol or ethanol to form fatty acid alkyl esters and glycerol. Conditions typically include moderate temperatures around 50–60°C for methanol; higher for ethanol, and the use of base catalysts such as NaOH or KOH. Free fatty acids can react with base to form soaps, reducing yield; pretreatment or acid catalysis may be used.

Other uses: Transesterification is also employed in polymer chemistry to exchange ester groups between molecules, enabling polymer modification, recycling of polyesters, and synthesis of oligomers. Enzymatic transesterification using lipases offers mild conditions and high selectivity, at the expense of rate and cost.

Overview: The reaction is central in renewable fuel production and in ester chemistry. Catalyst choice, feedstock composition, and alcohol-to-ester stoichiometry influence rate, conversion, and by-products. Safety considerations include handling of methanol and catalysts; environmental aspects relate to energy balance and waste glycerol management.