Acetylation in the Absence of a Specifically Added Catalyst

The preparation of the first starch acetate, as well as the first cellulose acetate, was announced by SchUtzenberger in 1865. These acetates were prepared by heating the carbohydrates in acetic anhydride to about 140-160 . Further examination of this reaction has been made by Traquair who found that on heating starch to 90° with acetic anhydride a derivative of low acetyl content (1-4%) is obtained which is capable of forming clear, somewhat elastic films. This starch acetate, termed Feculose, was produced commercially for a time, being sold for use as a thickening agent and as a size for textiles and paper. 

Elevated temperatures are apparently required to cause acetylation of untreated starches by acetic anhydride. Wheat starch, for example, is not acetylated by this reagent at 35°. At temperatures sufficiently high to produce acetylation, however, acetic anhydride causes extensive degradation of the starch molecules, resulting at 95° in the production of considerable quantities of maltose octaacetate.  

Ketene, alone or in the presence of an acid catalyst, has been ob-served to produce a low degree of acetylation in whole starch. 

The properties displayed by a starch triacetate, or by other starch esters are dependent upon the type and molecular weight of the starch or starch fraction used for the esterification. The properties may also be influenced by trace constituents or impurities such as compounds of nitrogen and phosphorus which are often observed in starch esters. While the nature of the nitrogenous constituent is not known, it has been demonstrated that much of the phosphorus of potato starch exists as a natural phosphoric ester of the amylopectin fraction. It is not surprising, therefore, that the phosphorus of potato starch is carried through an esterification and may influence the properties of the starch ester. The exact modifying effects of these components on the properties of starch esters, however, are still to be explained. 

Starch acetates may be characterized, in part, by (1) acetyl content, (2) solubility, (3) behavior in solution, (4) optical rotation, (5) fusion temperature, and (6) mechanical properties. 

Starch acetates may be characterized, in part, by (1) acetyl content, 

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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. 

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