Revisiting the mechanisms of low-temperature, base-catalysed ester interchange reactions

Ester interchange reactions such as the interesterification of triglycerides and their transesterification with methanol (methanolysis) to produce FAME (biodiesel) nowadays invariably use a basic catalyst such as an alkali alcoholate or hydroxide. Whereas it was formerly assumed that the catalytically active intermediates in the interesterification reaction and the methanolysis reaction were the glycerolate anion and the methanolate anion respectively, it now looks far more likely that the enolate anion plays a major role whenever the concentration of free alcohol groups in the reaction medium is small in comparison with the concentration of fatty acid moieties. Which mechanism dominates in which reaction and which reaction stage will be explained and discussed.