How is chemical interesterification initiated: Nucleophilic substitution or α‐proton abstraction?

Esters of carboxylic acids including 2‐methylhexanoic, 2‐methylbutyric, 2,2‐dimethyl‐4‐pentenoic, palmitic, and oleic acids were tested as substrates in methoxide‐catalyzed interesterification and transesterification. The aliphatic acid esters participated in the ester‐ester interchange upon addition of catalytic sodium methoxide. Their isopropyl esters also produced methyl esters on heating with sodium methoxide. The esters of α‐methyl‐substituted acids did not participate in the ester‐ester interchange. Their isopropyl esters did not react with methoxide to produce methyl esters. However, upon addition of methanol with sodium methoxide, their methyl esters were produced. These results indicate that the first step in interesterification is possibly that methoxide abstracts the α‐hydrogen of an ester to form a carbanion. Interesterification is then likely completed via a Claisen condensation mechanism involving the β‐keto ester anion as the active intermediate. The β‐keto ester anion contains positively charged ketone and acyl carbons that are active sites for nucleophilic attack by anions such as methoxide and glycerinate, which would produce a methyl ester or rearrange acyls randomly. On the other hand, transesterification is a nucleophilic substitution by methoxide at the acyl carbon in the presence of methanol.