Acylation of starch using trifluoroacetic anhydride promoter

The use of trifluoroacetic anhydride as a promoter for the acylation of granular starch is reported. Efficient acylation in a homogeneous solution is attained when the carboxylic acid of interest and trifluoroacetic anhydride are added, the latter in at least twice molar excess to the hydroxyl groups of the anhydroglucose residues. The reaction necessitates prior heating of granular starch in acetic acid before the acylation reagent is added. The product is an acylated starch with degrees of substitution (DS) of 2 to 3, as analyzed by gas-liquid chromatography or high-pressure liquid chromatography. Lower molar ratios of trifluoroacetic anhydride to the anhydroglucose residue gave lower degree of substitution. Additionally, starch acetates with different degree of substitution are prepared by addition of limited amounts of the acylation reagent or by partial de-acetylation of starch triacetate with sodium methoxide. Starch esters are prepared from acetic, butanoic, hexanoic, octanoic, palmitic and benzoic acids.