Mechanistic Considerations of Efficient Esterification of Starch with Propionic Anhydride/Lauric Acid in the Green Solvent Imidazole

Abstract

Starch esters have become increasingly attractive as sustainable materials. Instead of carboxylic acid chlorides, mixtures of carboxylic acid anhydrides and carboxylic acids are used to synthesize highly substituted starch esters efficiently applying the new starch solvent imidazole. The reaction is a promising alternative to the existing protocols and leads to thermoplastic mixed starch esters. The mechanism for the reaction path is elucidated by the conversion of starch with propionic anhydride/lauric acid in molten imidazole and various model reactions applying Fourier transform infrared spectroscopy and NMR spectroscopic measurements. Mixed starch esters are obtained bearing not only propionate groups introduced by the reactive anhydride but also moieties originating from the lauric acid. The propionic anhydride is shown to react with imidazole to form the corresponding imidazolide. Moreover, the imidazolide is in equilibrium with the lauric acid to form the corresponding imidazolide as well.