Acetylation in the Presence of a Specifically Added Catalyst, Other than Pyridine

Acetylation in the Presence of a Specifically Added Catalyst, Other than Pyridine 

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 

While whole starch is very slowly or incompletely dissolved by mixtures of acetic acid, acetic anhydride, and sulfuric acid, the acetyl content of the dissolved portion rises rapidly with increased amounts of sulfuric acid or the application of higher temperatures. In contrast to whole starch a finely divided, fluffy starch, prepared as described in Section II, might be expected to be acetylated rapidly and completely by mixtures of acetic acid, acetic anhydride, and sulfuric acid. 

Acetic anhydride and sulfuric acid together also form a powerful acetylating mixture capable of esterifying starch at low temperatures. Pregl treated a Zulkowsky heat-modified potato starch with a mixture of cold acetic anhydride and sulfuric acid. After 40 hours, most of the starch dissolved to produce a product containing 46.7-47.4% acetyl. 

Peiser - has pretreated wheat starch by gelatinization in water, precipitation by alcohol and drying. Acetylation was effected in the cold with 6 parts of acetic anhydride and 0.05 part of sulfuric acid. After eight hours, only 14% of the starch dissolved to produce an acetate containing 46.7% acetyl. At 55 , the acetyl content of the dissolved starch is reported as 47.3%. On the other hand, Haworth, Hirst and Webb on following Peiser s procedure have not obtained a similar product. 

---