Acylation of starch in the presence of carboxylic acids and amides as organocatalysts and as hydrogen bond donors in deep eutectic solvents

This study aims to investigate starch acylation using a new type of green solvents, namely deep eutectic solvents (DES), consisting of choline chloride (ChCl) as a hydrogen bond acceptor (HBA) and a series of carboxylic acids (citric, tartaric, malic, lactic, oxalic) or amides (urea, thiourea) as organocatalysts and hydrogen bond donors; and to compare acids versus amides as organocatalysts. The substitution degree (DS), determined by ¹H NMR and confirmed by FTIR and ¹³C NMR, increased from 1.49 (reaction yield of 63.2 %) to 2.22 (reaction yield of 69.4 %) with increasing acetic anhydride quantity, and from 0.13 (69.3 %) to 2.03 (59.5 %) with temperature increasing from 90 °C to 110 °C. Spectroscopic analysis revealed the presence of carbamate byproducts, in the samples acylated in the presence of DESs with urea/thiourea. X-ray diffraction (XRD) patterns showed that the structure of acylated samples differs in acid catalysis compared to basic catalysis, from amorphous to crystalline, respectively, and the crystallinity degree decreased with DS.