31 июля, 2015 
Malyar Esterification of cellulose to give cellulose trinitrate was discovered by Schonbein in 1846 using a mixture of sulfuric and nitric acids. The resultant compound was so flammable that its first use was as smokeless gunpowder. By the end of the ninteenth century, cellulose nitrates had been prepared with a lower DS, and they could safely be used for other purposes. All cellulose nitrates are prepared by Schonbein’s method, in which an aqueous slurry of cellulose is reacted with nitric acid in the presence of sulfuric acid (Fig. 2a). The reaction is in equilibrium and thus the removal of water during the reaction forces the reaction to completion [3]. The relative concentrations of the reacting species determine the ultimate DS that can be obtained. Boiling the nitrated product in water removes sulfate groups that can make the cellulose nitrate unstable, and digestion may be the final preparation step if a lower-viscosity material is desired.
The discovery that cellulose esters could be prepared with organic substituents led to the development of cellulose derivatives that had decreased flammability compared to that of cellulose nitrate. The most important organic ester is cellulose acetate. It is prepared by the reaction of acetic anhydride on cellulose in the presence of sulfuric acid. Acetic acid is
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Figure 2 Major reactions of cellulose: (a) nitration by nitric acid; (b) esterification by acetic anhydride; (c) hydrolysis of tertiary cellulose acetate by hydrochloric acid; (d) deprotonation of cellulose, the first step in etherification; (e) etherification of sodium cellulose by an alkyl halide; (f) etherification of sodium cellulose by an epoxide. |
used as the solvent and the reaction is carried out for about 8 h to yield the triester (defined as having a DS greater than 2.75) (Fig. 2b). The derivatives with lower DS values are obtained by the hydrolysis of the triester by hydrochloric acid to yield the desired substitution (Fig. 2c).
Esters of other aliphatic acids are prepared in a similar manner using the appropriate anhydride. Industrially important esters include cellulose propionate, cellulose butyrate, cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB). The mixed esters, CAP and CAB, are prepared by using a mixture of anhydrides in the desired ratios, or by reacting cellulose with propionic or butyric acid and acetic anhydride. Both reactions require sulfuric acid as a catalyst.
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