Catalysts, for acetylation of starch

Catalysis, enzyme-substrate and intermediate compound theory in homo-and heterogeneous, V, 51 Catalysts, for acetonation, III, 51 for acetylation of starch, I, 284, 286 Bourguel s, II, 109, 110, 113 for esterification of cellulose, I, 312 in oxidation of carbohydrates by halogens, III, 177... 

Pyridine is by far the most commonly used catalyst or accessory in the present day acetylations of starch. It is particularly suitable for the laboratory acetylation of starch in those cases where damage to the basic molecular structure is to be minimized. 

Methods for the preparation of starch acetates may be classified as acetylation (1) in the presence of pyridine, (2) in the presence of specifically added catalysts other than pyridine, and (3) in the absence of a specifically added catalyst. Methods classified in (1) produce little or no degradation of the starch molecules. Those in (2) may or may not produce degradation, depending upon the reagents selected, while methods in (3) almost invariably cause a depolymerization of the starch. 

Sulfuric acid is a powerful esterification catalyst. It has been widely applied with mixtures of acetic acid and acetic anhydride to promote acetylations of numerous substances. Use of this catalyzed reaction for starch acetylation, however, has not risen to pre-eminence among starch acetylation methods as it has done among cellulose acetylations, although both reactions were discovered at the same time. The underdevelopment of this reaction in the starch field may be due to the following causes (1) sulfuric acid, a powerful acetylation catalyst, strongly catalyzes the hydrolysis of starch molecules and cannot be used for starch acetylations in the concentrations found most effective for cellulose reactions (2) most investigations of this reaction have been made on whole granules... 

Acid catalysts other than sulfuric acid have been tested with acetic anhydride for use in starch acetylation. None of these, however, seem to exert as powerful a catalytic action as sulfuric acid, and some, particularly the halogen acids, cause even more extensive depolymerization of the starch. For example, acetic anhydride saturated with hydrogen chloride rapidly dissolves starch and converts it to low molec-... 

Zinc chloride, although commonly used as a catalyst in the acetylation of simple sugars, has not been extensively investigated as a catalytic agent for starch acetylation. Law has reported that he could not acetylate ordinary starch with a mixture of zinc chloride, acetic acid, and acetic anhydride. Starch heated in glycerol at 100-170 , and therefore probably degraded, has been acetylated, however, in a mixture of zinc chloride and acetic anhydride at 50-80 . 

Sulfuryl chloride and magnesium perchlorate have been suggested as acetylation catalysts for starch. Sulfur dioxide in acetic acid/ sulfur trioxide in acetic anhydride, and sulfonated fatty acid or sulfonated salicylic acid in a mixture of acetic anhydride and acetic acid have also been reported to acetylate starch. 

Common starch esters include the acetates. High d.s. starch acetates have been formed by using acetic anhydride with either sodium acetate or pyridine catalysts at 90-100 °C [158,159,160]. The major use of starch acetates have been in the study of the stmctures of amylose and amylopectin after acid hydrolysis. Acetylation of granular starch in aqueous suspension by acetic anhydride at pH 10-11 is used to produce low d.s. starch acetates that are primarily used for the stabilization of their viscosity and for their water-soluble clarity. The acetylation decreases the hydrophilic character and increases the hydrophobic character of starch. The major uses of starch acetates is in the paper industry for surface sizing to give improved print quality, uniform porosity, surface strength, and resistance to various solvents [161]. 

Starch has been acetylated by Barnett s mixture, W hich is composed of acetic acid, acetic anhydride, and a catalyst consisting of equal parts of sulfur dioxide and chlorine. This mixture, originally developed for cellulose esterification," readily acetylates starch at 55 , the acetyl content reaching the theoretical value of 44.8% in about four hours. A critical evaluation of the reaction by Hassid and Dore, and particularly by Higginbotham and Richardson, has shown that the acetates prepared have the viscosity, reducing power, and solubility characteristics indicative of a partially degraded starch. 

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